| Message ID | 1451572003-2440-23-git-send-email-mst@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > This defines __smp_xxx barriers for s390, > for use by virtualization. > > Some smp_xxx barriers are removed as they are > defined correctly by asm-generic/barriers.h > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > unconditionally on this architecture. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > Acked-by: Arnd Bergmann <arnd@arndb.de> > --- > arch/s390/include/asm/barrier.h | 15 +++++++++------ > 1 file changed, 9 insertions(+), 6 deletions(-) > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > index c358c31..fbd25b2 100644 > --- a/arch/s390/include/asm/barrier.h > +++ b/arch/s390/include/asm/barrier.h > @@ -26,18 +26,21 @@ > #define wmb() barrier() > #define dma_rmb() mb() > #define dma_wmb() mb() > -#define smp_mb() mb() > -#define smp_rmb() rmb() > -#define smp_wmb() wmb() > - > -#define smp_store_release(p, v) \ > +#define __smp_mb() mb() > +#define __smp_rmb() rmb() > +#define __smp_wmb() wmb() > +#define smp_mb() __smp_mb() > +#define smp_rmb() __smp_rmb() > +#define smp_wmb() __smp_wmb() Why define the smp_*mb() primitives here? Would not the inclusion of asm-generic/barrier.h do this?
On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > This defines __smp_xxx barriers for s390, > > for use by virtualization. > > > > Some smp_xxx barriers are removed as they are > > defined correctly by asm-generic/barriers.h > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > unconditionally on this architecture. > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > --- > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > index c358c31..fbd25b2 100644 > > --- a/arch/s390/include/asm/barrier.h > > +++ b/arch/s390/include/asm/barrier.h > > @@ -26,18 +26,21 @@ > > #define wmb() barrier() > > #define dma_rmb() mb() > > #define dma_wmb() mb() > > -#define smp_mb() mb() > > -#define smp_rmb() rmb() > > -#define smp_wmb() wmb() > > - > > -#define smp_store_release(p, v) \ > > +#define __smp_mb() mb() > > +#define __smp_rmb() rmb() > > +#define __smp_wmb() wmb() > > +#define smp_mb() __smp_mb() > > +#define smp_rmb() __smp_rmb() > > +#define smp_wmb() __smp_wmb() > > Why define the smp_*mb() primitives here? Would not the inclusion of > asm-generic/barrier.h do this? No because the generic one is a nop on !SMP, this one isn't. Pls note this patch is just reordering code without making functional changes. And at the moment, on s390 smp_xxx barriers are always non empty. Some of this could be sub-optimal, but since on s390 Linux always runs on a hypervisor, I am not sure it's safe to use the generic version - in other words, it just might be that for s390 smp_ and virt_ barriers must be equivalent. If in fact this turns out to be wrong, I can pick up a patch to change this, but I'd rather make this a patch on top so that my patches are testable just by compiling and comparing the binary.
On Mon, 4 Jan 2016 22:18:58 +0200 "Michael S. Tsirkin" <mst@redhat.com> wrote: > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > This defines __smp_xxx barriers for s390, > > > for use by virtualization. > > > > > > Some smp_xxx barriers are removed as they are > > > defined correctly by asm-generic/barriers.h > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > unconditionally on this architecture. > > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > > --- > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > index c358c31..fbd25b2 100644 > > > --- a/arch/s390/include/asm/barrier.h > > > +++ b/arch/s390/include/asm/barrier.h > > > @@ -26,18 +26,21 @@ > > > #define wmb() barrier() > > > #define dma_rmb() mb() > > > #define dma_wmb() mb() > > > -#define smp_mb() mb() > > > -#define smp_rmb() rmb() > > > -#define smp_wmb() wmb() > > > - > > > -#define smp_store_release(p, v) \ > > > +#define __smp_mb() mb() > > > +#define __smp_rmb() rmb() > > > +#define __smp_wmb() wmb() > > > +#define smp_mb() __smp_mb() > > > +#define smp_rmb() __smp_rmb() > > > +#define smp_wmb() __smp_wmb() > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > asm-generic/barrier.h do this? > > No because the generic one is a nop on !SMP, this one isn't. > > Pls note this patch is just reordering code without making > functional changes. > And at the moment, on s390 smp_xxx barriers are always non empty. The s390 kernel is SMP to 99.99%, we just didn't bother with a non-smp variant for the memory-barriers. If the generic header is used we'd get the non-smp version for free. It will save a small amount of text space for CONFIG_SMP=n. > Some of this could be sub-optimal, but > since on s390 Linux always runs on a hypervisor, > I am not sure it's safe to use the generic version - > in other words, it just might be that for s390 smp_ and virt_ > barriers must be equivalent. The definition of the memory barriers is independent from the fact if the system is running on an hypervisor or not. Is there really an architecture where you need special virt_xxx barriers?!?
On Tue, Jan 05, 2016 at 09:13:19AM +0100, Martin Schwidefsky wrote: > On Mon, 4 Jan 2016 22:18:58 +0200 > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > > This defines __smp_xxx barriers for s390, > > > > for use by virtualization. > > > > > > > > Some smp_xxx barriers are removed as they are > > > > defined correctly by asm-generic/barriers.h > > > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > > unconditionally on this architecture. > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > > > --- > > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > > index c358c31..fbd25b2 100644 > > > > --- a/arch/s390/include/asm/barrier.h > > > > +++ b/arch/s390/include/asm/barrier.h > > > > @@ -26,18 +26,21 @@ > > > > #define wmb() barrier() > > > > #define dma_rmb() mb() > > > > #define dma_wmb() mb() > > > > -#define smp_mb() mb() > > > > -#define smp_rmb() rmb() > > > > -#define smp_wmb() wmb() > > > > - > > > > -#define smp_store_release(p, v) \ > > > > +#define __smp_mb() mb() > > > > +#define __smp_rmb() rmb() > > > > +#define __smp_wmb() wmb() > > > > +#define smp_mb() __smp_mb() > > > > +#define smp_rmb() __smp_rmb() > > > > +#define smp_wmb() __smp_wmb() > > > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > > asm-generic/barrier.h do this? > > > > No because the generic one is a nop on !SMP, this one isn't. > > > > Pls note this patch is just reordering code without making > > functional changes. > > And at the moment, on s390 smp_xxx barriers are always non empty. > > The s390 kernel is SMP to 99.99%, we just didn't bother with a > non-smp variant for the memory-barriers. If the generic header > is used we'd get the non-smp version for free. It will save a > small amount of text space for CONFIG_SMP=n. OK, so I'll queue a patch to do this then? Just to make sure: the question would be, are smp_xxx barriers ever used in s390 arch specific code to flush in/out memory accesses for synchronization with the hypervisor? I went over s390 arch code and it seems to me the answer is no (except of course for virtio). But I also see a lot of weirdness on this architecture. I found these calls: arch/s390/include/asm/bitops.h: smp_mb__before_atomic(); arch/s390/include/asm/bitops.h: smp_mb(); Not used in arch specific code so this is likely OK. arch/s390/kernel/vdso.c: smp_mb(); Looking at Author: Christian Borntraeger <borntraeger@de.ibm.com> Date: Fri Sep 11 16:23:06 2015 +0200 s390/vdso: use correct memory barrier By definition smp_wmb only orders writes against writes. (Finish all previous writes, and do not start any future write). To protect the vdso init code against early reads on other CPUs, let's use a full smp_mb at the end of vdso init. As right now smp_wmb is implemented as full serialization, this needs no stable backport, but this change will be necessary if we reimplement smp_wmb. ok from hypervisor point of view, but it's also strange: 1. why isn't this paired with another mb somewhere? this seems to violate barrier pairing rules. 2. how does smp_mb protect against early reads on other CPUs? It normally does not: it orders reads from this CPU versus writes from same CPU. But init code does not appear to read anything. Maybe this is some s390 specific trick? I could not figure out the above commit. arch/s390/kvm/kvm-s390.c: smp_mb(); Does not appear to be paired with anything. arch/s390/lib/spinlock.c: smp_mb(); arch/s390/lib/spinlock.c: smp_mb(); Seems ok, and appears paired properly. Just to make sure - spinlock is not paravirtualized on s390, is it? rch/s390/kernel/time.c: smp_wmb(); arch/s390/kernel/time.c: smp_wmb(); arch/s390/kernel/time.c: smp_wmb(); arch/s390/kernel/time.c: smp_wmb(); It's all around vdso, so I'm guessing userspace is using this, this is why there's no pairing. > > Some of this could be sub-optimal, but > > since on s390 Linux always runs on a hypervisor, > > I am not sure it's safe to use the generic version - > > in other words, it just might be that for s390 smp_ and virt_ > > barriers must be equivalent. > > The definition of the memory barriers is independent from the fact > if the system is running on an hypervisor or not. > Is there really > an architecture where you need special virt_xxx barriers?!? It is whenever host and guest or two guests access memory at the same time. The optimization where smp_xxx barriers are compiled out when CONFIG_SMP is cleared means that two UP guests running on an SMP host can not use smp_xxx barriers for communication. See explanation here: http://thread.gmane.org/gmane.linux.kernel.virtualization/26555 > -- > blue skies, > Martin. > > "Reality continues to ruin my life." - Calvin.
On Tue, 5 Jan 2016 11:30:19 +0200 "Michael S. Tsirkin" <mst@redhat.com> wrote: > On Tue, Jan 05, 2016 at 09:13:19AM +0100, Martin Schwidefsky wrote: > > On Mon, 4 Jan 2016 22:18:58 +0200 > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > > > This defines __smp_xxx barriers for s390, > > > > > for use by virtualization. > > > > > > > > > > Some smp_xxx barriers are removed as they are > > > > > defined correctly by asm-generic/barriers.h > > > > > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > > > unconditionally on this architecture. > > > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > > > > --- > > > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > > > index c358c31..fbd25b2 100644 > > > > > --- a/arch/s390/include/asm/barrier.h > > > > > +++ b/arch/s390/include/asm/barrier.h > > > > > @@ -26,18 +26,21 @@ > > > > > #define wmb() barrier() > > > > > #define dma_rmb() mb() > > > > > #define dma_wmb() mb() > > > > > -#define smp_mb() mb() > > > > > -#define smp_rmb() rmb() > > > > > -#define smp_wmb() wmb() > > > > > - > > > > > -#define smp_store_release(p, v) \ > > > > > +#define __smp_mb() mb() > > > > > +#define __smp_rmb() rmb() > > > > > +#define __smp_wmb() wmb() > > > > > +#define smp_mb() __smp_mb() > > > > > +#define smp_rmb() __smp_rmb() > > > > > +#define smp_wmb() __smp_wmb() > > > > > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > > > asm-generic/barrier.h do this? > > > > > > No because the generic one is a nop on !SMP, this one isn't. > > > > > > Pls note this patch is just reordering code without making > > > functional changes. > > > And at the moment, on s390 smp_xxx barriers are always non empty. > > > > The s390 kernel is SMP to 99.99%, we just didn't bother with a > > non-smp variant for the memory-barriers. If the generic header > > is used we'd get the non-smp version for free. It will save a > > small amount of text space for CONFIG_SMP=n. > > OK, so I'll queue a patch to do this then? Yes please. > Just to make sure: the question would be, are smp_xxx barriers ever used > in s390 arch specific code to flush in/out memory accesses for > synchronization with the hypervisor? > > I went over s390 arch code and it seems to me the answer is no > (except of course for virtio). Correct. Guest to host communication either uses instructions which imply a memory barrier or QDIO which uses atomics. > But I also see a lot of weirdness on this architecture. Mostly historical, s390 actually is one of the easiest architectures in regard to memory barriers. > I found these calls: > > arch/s390/include/asm/bitops.h: smp_mb__before_atomic(); > arch/s390/include/asm/bitops.h: smp_mb(); > > Not used in arch specific code so this is likely OK. This has been introduced with git commit 5402ea6af11dc5a9, the smp_mb and smp_mb__before_atomic are used in clear_bit_unlock and __clear_bit_unlock which are 1:1 copies from the code in include/asm-generic/bitops/lock.h. Only test_and_set_bit_lock differs from the generic implementation. > arch/s390/kernel/vdso.c: smp_mb(); > > Looking at > Author: Christian Borntraeger <borntraeger@de.ibm.com> > Date: Fri Sep 11 16:23:06 2015 +0200 > > s390/vdso: use correct memory barrier > > By definition smp_wmb only orders writes against writes. (Finish all > previous writes, and do not start any future write). To protect the > vdso init code against early reads on other CPUs, let's use a full > smp_mb at the end of vdso init. As right now smp_wmb is implemented > as full serialization, this needs no stable backport, but this change > will be necessary if we reimplement smp_wmb. > > ok from hypervisor point of view, but it's also strange: > 1. why isn't this paired with another mb somewhere? > this seems to violate barrier pairing rules. > 2. how does smp_mb protect against early reads on other CPUs? > It normally does not: it orders reads from this CPU versus writes > from same CPU. But init code does not appear to read anything. > Maybe this is some s390 specific trick? > > I could not figure out the above commit. That smp_mb can be removed. The initial s390 vdso code is heavily influenced by the powerpc version which does have a smp_wmb in vdso_init right before the vdso_ready=1 assignment. s390 has no need for that. > > arch/s390/kvm/kvm-s390.c: smp_mb(); > > Does not appear to be paired with anything. This one does not make sense to me. Imho can be removed as well. > arch/s390/lib/spinlock.c: smp_mb(); > arch/s390/lib/spinlock.c: smp_mb(); > > Seems ok, and appears paired properly. > Just to make sure - spinlock is not paravirtualized on s390, is it? s390 just uses the compare-and-swap instruction for the basic lock/unlock operation, this implies the memory barrier. We do call the hypervisor for contended locks if the lock can not be acquired after a number of retries. A while ago we did play with ticket spinlocks but they behaved badly in out usual virtualized environments. If we find the time we might take a closer look at the para-virtualized queued spinlocks. > rch/s390/kernel/time.c: smp_wmb(); > arch/s390/kernel/time.c: smp_wmb(); > arch/s390/kernel/time.c: smp_wmb(); > arch/s390/kernel/time.c: smp_wmb(); > > It's all around vdso, so I'm guessing userspace is using this, > this is why there's no pairing. Correct, this is the update count mechanics with the vdso user space code. > > > Some of this could be sub-optimal, but > > > since on s390 Linux always runs on a hypervisor, > > > I am not sure it's safe to use the generic version - > > > in other words, it just might be that for s390 smp_ and virt_ > > > barriers must be equivalent. > > > > The definition of the memory barriers is independent from the fact > > if the system is running on an hypervisor or not. > > Is there really > > an architecture where you need special virt_xxx barriers?!? > > It is whenever host and guest or two guests access memory at > the same time. > > The optimization where smp_xxx barriers are compiled out when > CONFIG_SMP is cleared means that two UP guests running > on an SMP host can not use smp_xxx barriers for communication. > > See explanation here: > http://thread.gmane.org/gmane.linux.kernel.virtualization/26555 Got it, makes sense.
On Tue, Jan 05, 2016 at 01:08:52PM +0100, Martin Schwidefsky wrote: > On Tue, 5 Jan 2016 11:30:19 +0200 > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > On Tue, Jan 05, 2016 at 09:13:19AM +0100, Martin Schwidefsky wrote: > > > On Mon, 4 Jan 2016 22:18:58 +0200 > > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > > > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > > > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > > > > This defines __smp_xxx barriers for s390, > > > > > > for use by virtualization. > > > > > > > > > > > > Some smp_xxx barriers are removed as they are > > > > > > defined correctly by asm-generic/barriers.h > > > > > > > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > > > > unconditionally on this architecture. > > > > > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > > > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > > > > > --- > > > > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > > > > index c358c31..fbd25b2 100644 > > > > > > --- a/arch/s390/include/asm/barrier.h > > > > > > +++ b/arch/s390/include/asm/barrier.h > > > > > > @@ -26,18 +26,21 @@ > > > > > > #define wmb() barrier() > > > > > > #define dma_rmb() mb() > > > > > > #define dma_wmb() mb() > > > > > > -#define smp_mb() mb() > > > > > > -#define smp_rmb() rmb() > > > > > > -#define smp_wmb() wmb() > > > > > > - > > > > > > -#define smp_store_release(p, v) \ > > > > > > +#define __smp_mb() mb() > > > > > > +#define __smp_rmb() rmb() > > > > > > +#define __smp_wmb() wmb() > > > > > > +#define smp_mb() __smp_mb() > > > > > > +#define smp_rmb() __smp_rmb() > > > > > > +#define smp_wmb() __smp_wmb() > > > > > > > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > > > > asm-generic/barrier.h do this? > > > > > > > > No because the generic one is a nop on !SMP, this one isn't. > > > > > > > > Pls note this patch is just reordering code without making > > > > functional changes. > > > > And at the moment, on s390 smp_xxx barriers are always non empty. > > > > > > The s390 kernel is SMP to 99.99%, we just didn't bother with a > > > non-smp variant for the memory-barriers. If the generic header > > > is used we'd get the non-smp version for free. It will save a > > > small amount of text space for CONFIG_SMP=n. > > > > OK, so I'll queue a patch to do this then? > > Yes please. OK, I'll add a patch on top in v3. > > Just to make sure: the question would be, are smp_xxx barriers ever used > > in s390 arch specific code to flush in/out memory accesses for > > synchronization with the hypervisor? > > > > I went over s390 arch code and it seems to me the answer is no > > (except of course for virtio). > > Correct. Guest to host communication either uses instructions which > imply a memory barrier or QDIO which uses atomics. And atomics imply a barrier on s390, right? > > But I also see a lot of weirdness on this architecture. > > Mostly historical, s390 actually is one of the easiest architectures in > regard to memory barriers. > > > I found these calls: > > > > arch/s390/include/asm/bitops.h: smp_mb__before_atomic(); > > arch/s390/include/asm/bitops.h: smp_mb(); > > > > Not used in arch specific code so this is likely OK. > > This has been introduced with git commit 5402ea6af11dc5a9, the smp_mb > and smp_mb__before_atomic are used in clear_bit_unlock and > __clear_bit_unlock which are 1:1 copies from the code in > include/asm-generic/bitops/lock.h. Only test_and_set_bit_lock differs > from the generic implementation. something to keep in mind, but I'd rather not touch bitops at the moment - this patchset is already too big. > > arch/s390/kernel/vdso.c: smp_mb(); > > > > Looking at > > Author: Christian Borntraeger <borntraeger@de.ibm.com> > > Date: Fri Sep 11 16:23:06 2015 +0200 > > > > s390/vdso: use correct memory barrier > > > > By definition smp_wmb only orders writes against writes. (Finish all > > previous writes, and do not start any future write). To protect the > > vdso init code against early reads on other CPUs, let's use a full > > smp_mb at the end of vdso init. As right now smp_wmb is implemented > > as full serialization, this needs no stable backport, but this change > > will be necessary if we reimplement smp_wmb. > > > > ok from hypervisor point of view, but it's also strange: > > 1. why isn't this paired with another mb somewhere? > > this seems to violate barrier pairing rules. > > 2. how does smp_mb protect against early reads on other CPUs? > > It normally does not: it orders reads from this CPU versus writes > > from same CPU. But init code does not appear to read anything. > > Maybe this is some s390 specific trick? > > > > I could not figure out the above commit. > > That smp_mb can be removed. The initial s390 vdso code is heavily influenced > by the powerpc version which does have a smp_wmb in vdso_init right before > the vdso_ready=1 assignment. s390 has no need for that. > > > > > arch/s390/kvm/kvm-s390.c: smp_mb(); > > > > Does not appear to be paired with anything. > > This one does not make sense to me. Imho can be removed as well. > > > arch/s390/lib/spinlock.c: smp_mb(); > > arch/s390/lib/spinlock.c: smp_mb(); > > > > Seems ok, and appears paired properly. > > Just to make sure - spinlock is not paravirtualized on s390, is it? > > s390 just uses the compare-and-swap instruction for the basic lock/unlock > operation, this implies the memory barrier. We do call the hypervisor for > contended locks if the lock can not be acquired after a number of retries. > > A while ago we did play with ticket spinlocks but they behaved badly in > out usual virtualized environments. If we find the time we might take a > closer look at the para-virtualized queued spinlocks. > > > rch/s390/kernel/time.c: smp_wmb(); > > arch/s390/kernel/time.c: smp_wmb(); > > arch/s390/kernel/time.c: smp_wmb(); > > arch/s390/kernel/time.c: smp_wmb(); > > > > It's all around vdso, so I'm guessing userspace is using this, > > this is why there's no pairing. > > Correct, this is the update count mechanics with the vdso user space code. > > > > > Some of this could be sub-optimal, but > > > > since on s390 Linux always runs on a hypervisor, > > > > I am not sure it's safe to use the generic version - > > > > in other words, it just might be that for s390 smp_ and virt_ > > > > barriers must be equivalent. > > > > > > The definition of the memory barriers is independent from the fact > > > if the system is running on an hypervisor or not. > > > Is there really > > > an architecture where you need special virt_xxx barriers?!? > > > > It is whenever host and guest or two guests access memory at > > the same time. > > > > The optimization where smp_xxx barriers are compiled out when > > CONFIG_SMP is cleared means that two UP guests running > > on an SMP host can not use smp_xxx barriers for communication. > > > > See explanation here: > > http://thread.gmane.org/gmane.linux.kernel.virtualization/26555 > > Got it, makes sense. An ack would be appreciated. > -- > blue skies, > Martin. > > "Reality continues to ruin my life." - Calvin.
On Tue, 5 Jan 2016 15:04:43 +0200 "Michael S. Tsirkin" <mst@redhat.com> wrote: > On Tue, Jan 05, 2016 at 01:08:52PM +0100, Martin Schwidefsky wrote: > > On Tue, 5 Jan 2016 11:30:19 +0200 > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > On Tue, Jan 05, 2016 at 09:13:19AM +0100, Martin Schwidefsky wrote: > > > > On Mon, 4 Jan 2016 22:18:58 +0200 > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > > > > > > > On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote: > > > > > > On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote: > > > > > > > This defines __smp_xxx barriers for s390, > > > > > > > for use by virtualization. > > > > > > > > > > > > > > Some smp_xxx barriers are removed as they are > > > > > > > defined correctly by asm-generic/barriers.h > > > > > > > > > > > > > > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers > > > > > > > unconditionally on this architecture. > > > > > > > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > > > > > > Acked-by: Arnd Bergmann <arnd@arndb.de> > > > > > > > --- > > > > > > > arch/s390/include/asm/barrier.h | 15 +++++++++------ > > > > > > > 1 file changed, 9 insertions(+), 6 deletions(-) > > > > > > > > > > > > > > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h > > > > > > > index c358c31..fbd25b2 100644 > > > > > > > --- a/arch/s390/include/asm/barrier.h > > > > > > > +++ b/arch/s390/include/asm/barrier.h > > > > > > > @@ -26,18 +26,21 @@ > > > > > > > #define wmb() barrier() > > > > > > > #define dma_rmb() mb() > > > > > > > #define dma_wmb() mb() > > > > > > > -#define smp_mb() mb() > > > > > > > -#define smp_rmb() rmb() > > > > > > > -#define smp_wmb() wmb() > > > > > > > - > > > > > > > -#define smp_store_release(p, v) \ > > > > > > > +#define __smp_mb() mb() > > > > > > > +#define __smp_rmb() rmb() > > > > > > > +#define __smp_wmb() wmb() > > > > > > > +#define smp_mb() __smp_mb() > > > > > > > +#define smp_rmb() __smp_rmb() > > > > > > > +#define smp_wmb() __smp_wmb() > > > > > > > > > > > > Why define the smp_*mb() primitives here? Would not the inclusion of > > > > > > asm-generic/barrier.h do this? > > > > > > > > > > No because the generic one is a nop on !SMP, this one isn't. > > > > > > > > > > Pls note this patch is just reordering code without making > > > > > functional changes. > > > > > And at the moment, on s390 smp_xxx barriers are always non empty. > > > > > > > > The s390 kernel is SMP to 99.99%, we just didn't bother with a > > > > non-smp variant for the memory-barriers. If the generic header > > > > is used we'd get the non-smp version for free. It will save a > > > > small amount of text space for CONFIG_SMP=n. > > > > > > OK, so I'll queue a patch to do this then? > > > > Yes please. > > OK, I'll add a patch on top in v3. Good, with this addition: Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com> > > > Just to make sure: the question would be, are smp_xxx barriers ever used > > > in s390 arch specific code to flush in/out memory accesses for > > > synchronization with the hypervisor? > > > > > > I went over s390 arch code and it seems to me the answer is no > > > (except of course for virtio). > > > > Correct. Guest to host communication either uses instructions which > > imply a memory barrier or QDIO which uses atomics. > > And atomics imply a barrier on s390, right? Yes they do. > > > But I also see a lot of weirdness on this architecture. > > > > Mostly historical, s390 actually is one of the easiest architectures in > > regard to memory barriers. > > > > > I found these calls: > > > > > > arch/s390/include/asm/bitops.h: smp_mb__before_atomic(); > > > arch/s390/include/asm/bitops.h: smp_mb(); > > > > > > Not used in arch specific code so this is likely OK. > > > > This has been introduced with git commit 5402ea6af11dc5a9, the smp_mb > > and smp_mb__before_atomic are used in clear_bit_unlock and > > __clear_bit_unlock which are 1:1 copies from the code in > > include/asm-generic/bitops/lock.h. Only test_and_set_bit_lock differs > > from the generic implementation. > > something to keep in mind, but > I'd rather not touch bitops at the moment - this patchset is already too big. With the conversion smp_mb__before_atomic to a barrier() it does the correct thing. I don't think that any chance is necessary.
On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote: > > arch/s390/kernel/vdso.c: smp_mb(); > > Looking at > Author: Christian Borntraeger <borntraeger@de.ibm.com> > Date: Fri Sep 11 16:23:06 2015 +0200 > > s390/vdso: use correct memory barrier > > By definition smp_wmb only orders writes against writes. (Finish all > previous writes, and do not start any future write). To protect the > vdso init code against early reads on other CPUs, let's use a full > smp_mb at the end of vdso init. As right now smp_wmb is implemented > as full serialization, this needs no stable backport, but this change > will be necessary if we reimplement smp_wmb. > > ok from hypervisor point of view, but it's also strange: > 1. why isn't this paired with another mb somewhere? > this seems to violate barrier pairing rules. > 2. how does smp_mb protect against early reads on other CPUs? > It normally does not: it orders reads from this CPU versus writes > from same CPU. But init code does not appear to read anything. > Maybe this is some s390 specific trick? > > I could not figure out the above commit. It was probably me misreading the code. I change a wmb into a full mb here since I was changing the defintion of wmb to a compiler barrier. I tried to fixup all users of wmb that really pair with other code. I assumed that there must be some reader (as there was a wmb before) but I could not figure out which. So I just played safe here. But it probably can be removed. > arch/s390/kvm/kvm-s390.c: smp_mb(); This can go. If you have a patch, I can carry that via the kvms390 tree, or I will spin a new patch with you as suggested-by. Christian
On Tue, Jan 05, 2016 at 04:39:37PM +0100, Christian Borntraeger wrote: > On 01/05/2016 10:30 AM, Michael S. Tsirkin wrote: > > > > > arch/s390/kernel/vdso.c: smp_mb(); > > > > Looking at > > Author: Christian Borntraeger <borntraeger@de.ibm.com> > > Date: Fri Sep 11 16:23:06 2015 +0200 > > > > s390/vdso: use correct memory barrier > > > > By definition smp_wmb only orders writes against writes. (Finish all > > previous writes, and do not start any future write). To protect the > > vdso init code against early reads on other CPUs, let's use a full > > smp_mb at the end of vdso init. As right now smp_wmb is implemented > > as full serialization, this needs no stable backport, but this change > > will be necessary if we reimplement smp_wmb. > > > > ok from hypervisor point of view, but it's also strange: > > 1. why isn't this paired with another mb somewhere? > > this seems to violate barrier pairing rules. > > 2. how does smp_mb protect against early reads on other CPUs? > > It normally does not: it orders reads from this CPU versus writes > > from same CPU. But init code does not appear to read anything. > > Maybe this is some s390 specific trick? > > > > I could not figure out the above commit. > > It was probably me misreading the code. I change a wmb into a full mb here > since I was changing the defintion of wmb to a compiler barrier. I tried to > fixup all users of wmb that really pair with other code. I assumed that there > must be some reader (as there was a wmb before) but I could not figure out > which. So I just played safe here. > > But it probably can be removed. > > > arch/s390/kvm/kvm-s390.c: smp_mb(); > > This can go. If you have a patch, I can carry that via the kvms390 tree, > or I will spin a new patch with you as suggested-by. > > Christian I have both, will post shortly.
diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h index c358c31..fbd25b2 100644 --- a/arch/s390/include/asm/barrier.h +++ b/arch/s390/include/asm/barrier.h @@ -26,18 +26,21 @@ #define wmb() barrier() #define dma_rmb() mb() #define dma_wmb() mb() -#define smp_mb() mb() -#define smp_rmb() rmb() -#define smp_wmb() wmb() - -#define smp_store_release(p, v) \ +#define __smp_mb() mb() +#define __smp_rmb() rmb() +#define __smp_wmb() wmb() +#define smp_mb() __smp_mb() +#define smp_rmb() __smp_rmb() +#define smp_wmb() __smp_wmb() + +#define __smp_store_release(p, v) \ do { \ compiletime_assert_atomic_type(*p); \ barrier(); \ WRITE_ONCE(*p, v); \ } while (0) -#define smp_load_acquire(p) \ +#define __smp_load_acquire(p) \ ({ \ typeof(*p) ___p1 = READ_ONCE(*p); \ compiletime_assert_atomic_type(*p); \