| Message ID | 1451572003-2440-34-git-send-email-mst@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 31/12/15 19:10, Michael S. Tsirkin wrote: > drivers/xen/xenbus/xenbus_comms.c uses > full memory barriers to communicate with the other side. > > For guests compiled with CONFIG_SMP, smp_wmb and smp_mb > would be sufficient, so mb() and wmb() here are only needed if > a non-SMP guest runs on an SMP host. > > Switch to virt_xxx barriers which serve this exact purpose. Acked-by: David Vrabel <david.vrabel@citrix.com> If you're feeling particularly keen there's a rmb() consume_one_event() in drivers/xen/events/events_fifo.c that can be converted to virt_rmb() as well. David
On Thu, 31 Dec 2015, Michael S. Tsirkin wrote: > drivers/xen/xenbus/xenbus_comms.c uses > full memory barriers to communicate with the other side. > > For guests compiled with CONFIG_SMP, smp_wmb and smp_mb > would be sufficient, so mb() and wmb() here are only needed if > a non-SMP guest runs on an SMP host. > > Switch to virt_xxx barriers which serve this exact purpose. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com> Are you also going to take care of drivers/xen/grant-table.c drivers/xen/evtchn.c drivers/xen/events/events_fifo.c drivers/xen/xen-scsiback.c drivers/xen/tmem.c drivers/xen/xen-pciback/pci_stub.c drivers/xen/xen-pciback/pciback_ops.c ? > drivers/xen/xenbus/xenbus_comms.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c > index fdb0f33..ecdecce 100644 > --- a/drivers/xen/xenbus/xenbus_comms.c > +++ b/drivers/xen/xenbus/xenbus_comms.c > @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len) > avail = len; > > /* Must write data /after/ reading the consumer index. */ > - mb(); > + virt_mb(); > > memcpy(dst, data, avail); > data += avail; > len -= avail; > > /* Other side must not see new producer until data is there. */ > - wmb(); > + virt_wmb(); > intf->req_prod += avail; > > /* Implies mb(): other side will see the updated producer. */ > @@ -180,14 +180,14 @@ int xb_read(void *data, unsigned len) > avail = len; > > /* Must read data /after/ reading the producer index. */ > - rmb(); > + virt_rmb(); > > memcpy(data, src, avail); > data += avail; > len -= avail; > > /* Other side must not see free space until we've copied out */ > - mb(); > + virt_mb(); > intf->rsp_cons += avail; > > pr_debug("Finished read of %i bytes (%i to go)\n", avail, len); > -- > MST >
On Thu, Dec 31, 2015 at 09:10:01PM +0200, Michael S. Tsirkin wrote: > drivers/xen/xenbus/xenbus_comms.c uses > full memory barriers to communicate with the other side. > > For guests compiled with CONFIG_SMP, smp_wmb and smp_mb > would be sufficient, so mb() and wmb() here are only needed if > a non-SMP guest runs on an SMP host. > > Switch to virt_xxx barriers which serve this exact purpose. > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > --- > drivers/xen/xenbus/xenbus_comms.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c > index fdb0f33..ecdecce 100644 > --- a/drivers/xen/xenbus/xenbus_comms.c > +++ b/drivers/xen/xenbus/xenbus_comms.c > @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len) > avail = len; > > /* Must write data /after/ reading the consumer index. */ > - mb(); > + virt_mb(); > So its possible to remove this barrier entirely, see the "CONTROL DEPENDNCIES" chunk of memory-barrier.txt. But do that in a separate patch series and only if you really really need the performance.
diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c index fdb0f33..ecdecce 100644 --- a/drivers/xen/xenbus/xenbus_comms.c +++ b/drivers/xen/xenbus/xenbus_comms.c @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len) avail = len; /* Must write data /after/ reading the consumer index. */ - mb(); + virt_mb(); memcpy(dst, data, avail); data += avail; len -= avail; /* Other side must not see new producer until data is there. */ - wmb(); + virt_wmb(); intf->req_prod += avail; /* Implies mb(): other side will see the updated producer. */ @@ -180,14 +180,14 @@ int xb_read(void *data, unsigned len) avail = len; /* Must read data /after/ reading the producer index. */ - rmb(); + virt_rmb(); memcpy(data, src, avail); data += avail; len -= avail; /* Other side must not see free space until we've copied out */ - mb(); + virt_mb(); intf->rsp_cons += avail; pr_debug("Finished read of %i bytes (%i to go)\n", avail, len);
drivers/xen/xenbus/xenbus_comms.c uses full memory barriers to communicate with the other side. For guests compiled with CONFIG_SMP, smp_wmb and smp_mb would be sufficient, so mb() and wmb() here are only needed if a non-SMP guest runs on an SMP host. Switch to virt_xxx barriers which serve this exact purpose. Signed-off-by: Michael S. Tsirkin <mst@redhat.com> --- drivers/xen/xenbus/xenbus_comms.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)