| Message ID | 25d5c6de22cabf6a4ecb44ce077f33aa5f10b4d4.1583547371.git.luto@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS | expand |
On Fri, Mar 6, 2020 at 6:16 PM Andy Lutomirski <luto@kernel.org> wrote: > > The ABI is broken and we cannot support it properly. Turn it off. > > If this causes a meaningful performance regression for someone, KVM > can introduce an improved ABI that is supportable. Ugh, disregard this patch. I flubbed the comment, and I'll send v2 shortly.
Andy Lutomirski <luto@kernel.org> writes: > The ABI is broken and we cannot support it properly. Turn it off. > > If this causes a meaningful performance regression for someone, KVM > can introduce an improved ABI that is supportable. > > Cc: stable@vger.kernel.org > Signed-off-by: Andy Lutomirski <luto@kernel.org> > --- > arch/x86/kernel/kvm.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c > index 93ab0cbd304e..71f9f39f93da 100644 > --- a/arch/x86/kernel/kvm.c > +++ b/arch/x86/kernel/kvm.c > @@ -318,11 +318,16 @@ static void kvm_guest_cpu_init(void) > > pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason)); > > -#ifdef CONFIG_PREEMPTION > - pa |= KVM_ASYNC_PF_SEND_ALWAYS; > -#endif > pa |= KVM_ASYNC_PF_ENABLED; > > + /* > + * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current > + * KVM paravirt ABI, if an async page fault occurs on an early > + * memory access in the normal (sync) #PF path or in an NMI > + * that happens early in the #PF code, the combination of CR2 > + * and the APF reason field will be corrupted. I don't think this can happen. In both cases IF == 0 and that async (think host side) page fault will be completely handled on the host. There is no injection happening in such a case ever. If it does, then yes the host side implementation is buggered, but AFAICT this is not the case. See also my reply in the other thread: https://lore.kernel.org/r/87r1y4a3gw.fsf@nanos.tec.linutronix.de Thanks, tglx
> On Mar 7, 2020, at 2:09 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > Andy Lutomirski <luto@kernel.org> writes: > >> The ABI is broken and we cannot support it properly. Turn it off. >> >> If this causes a meaningful performance regression for someone, KVM >> can introduce an improved ABI that is supportable. >> >> Cc: stable@vger.kernel.org >> Signed-off-by: Andy Lutomirski <luto@kernel.org> >> --- >> arch/x86/kernel/kvm.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c >> index 93ab0cbd304e..71f9f39f93da 100644 >> --- a/arch/x86/kernel/kvm.c >> +++ b/arch/x86/kernel/kvm.c >> @@ -318,11 +318,16 @@ static void kvm_guest_cpu_init(void) >> >> pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason)); >> >> -#ifdef CONFIG_PREEMPTION >> - pa |= KVM_ASYNC_PF_SEND_ALWAYS; >> -#endif >> pa |= KVM_ASYNC_PF_ENABLED; >> >> + /* >> + * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current >> + * KVM paravirt ABI, if an async page fault occurs on an early >> + * memory access in the normal (sync) #PF path or in an NMI >> + * that happens early in the #PF code, the combination of CR2 >> + * and the APF reason field will be corrupted. > > I don't think this can happen. In both cases IF == 0 and that async > (think host side) page fault will be completely handled on the > host. There is no injection happening in such a case ever. If it does, > then yes the host side implementation is buggered, but AFAICT this is > not the case. Indeed. But read v2 please. > > See also my reply in the other thread: > > https://lore.kernel.org/r/87r1y4a3gw.fsf@nanos.tec.linutronix.de > > Thanks, > > tglx
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 93ab0cbd304e..71f9f39f93da 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -318,11 +318,16 @@ static void kvm_guest_cpu_init(void) pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason)); -#ifdef CONFIG_PREEMPTION - pa |= KVM_ASYNC_PF_SEND_ALWAYS; -#endif pa |= KVM_ASYNC_PF_ENABLED; + /* + * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current + * KVM paravirt ABI, if an async page fault occurs on an early + * memory access in the normal (sync) #PF path or in an NMI + * that happens early in the #PF code, the combination of CR2 + * and the APF reason field will be corrupted. + */ + if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT)) pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
The ABI is broken and we cannot support it properly. Turn it off. If this causes a meaningful performance regression for someone, KVM can introduce an improved ABI that is supportable. Cc: stable@vger.kernel.org Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/x86/kernel/kvm.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)