vhost-net only uses memory barriers to control SMP effects
(communication with userspace potentially running on a different CPU),
so it should use SMP barriers and not mandatory barriers for memory
access ordering, as suggested by Documentation/memory-barriers.txt
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
drivers/vhost/vhost.c | 10 +++++-----
1 files changed, 5 insertions(+), 5 deletions(-)
The above applies on top of net-next-2.6. Does not seem to give any
measureable performance gain, but seems to generate less code
and I think it's better to use correct APIs.
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index c8c25db..6eb1525 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -685,7 +685,7 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct
vhost_log *log,
int i, r;
/* Make sure data written is seen before log. */
- wmb();
+ smp_wmb();
for (i = 0; i < log_num; ++i) {
u64 l = min(log[i].len, len);
r = log_write(vq->log_base, log[i].addr, l);
@@ -884,7 +884,7 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct
vhost_virtqueue *vq,
return vq->num;
/* Only get avail ring entries after they have been exposed by guest. */
- rmb();
+ smp_rmb();
/* Grab the next descriptor number they're advertising, and increment
* the index we've seen. */
@@ -996,14 +996,14 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned
int head, int len)
return -EFAULT;
}
/* Make sure buffer is written before we update index. */
- wmb();
+ smp_wmb();
if (put_user(vq->last_used_idx + 1, &vq->used->idx)) {
vq_err(vq, "Failed to increment used idx");
return -EFAULT;
}
if (unlikely(vq->log_used)) {
/* Make sure data is seen before log. */
- wmb();
+ smp_wmb();
log_write(vq->log_base, vq->log_addr + sizeof *vq->used->ring *
(vq->last_used_idx % vq->num),
sizeof *vq->used->ring);
@@ -1060,7 +1060,7 @@ bool vhost_enable_notify(struct vhost_virtqueue *vq)
}
/* They could have slipped one in as we were doing that: make
* sure it's written, then check again. */
- mb();
+ smp_mb();
r = get_user(avail_idx, &vq->avail->idx);
if (r) {
vq_err(vq, "Failed to check avail idx at %p: %d\n",
--
1.6.6.144.g5c3af
On Mon, Feb 01, 2010 at 07:21:02PM +0200, Michael S. Tsirkin wrote:> vhost-net only uses memory barriers to control SMP effects > (communication with userspace potentially running on a different CPU), > so it should use SMP barriers and not mandatory barriers for memory > access ordering, as suggested by Documentation/memory-barriers.txt > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>Rusty, any feedback on this one? Thanks!> --- > drivers/vhost/vhost.c | 10 +++++----- > 1 files changed, 5 insertions(+), 5 deletions(-) > > The above applies on top of net-next-2.6. Does not seem to give any > measureable performance gain, but seems to generate less code > and I think it's better to use correct APIs. > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index c8c25db..6eb1525 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -685,7 +685,7 @@ int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log, > int i, r; > > /* Make sure data written is seen before log. */ > - wmb(); > + smp_wmb(); > for (i = 0; i < log_num; ++i) { > u64 l = min(log[i].len, len); > r = log_write(vq->log_base, log[i].addr, l); > @@ -884,7 +884,7 @@ unsigned vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq, > return vq->num; > > /* Only get avail ring entries after they have been exposed by guest. */ > - rmb(); > + smp_rmb(); > > /* Grab the next descriptor number they're advertising, and increment > * the index we've seen. */ > @@ -996,14 +996,14 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len) > return -EFAULT; > } > /* Make sure buffer is written before we update index. */ > - wmb(); > + smp_wmb(); > if (put_user(vq->last_used_idx + 1, &vq->used->idx)) { > vq_err(vq, "Failed to increment used idx"); > return -EFAULT; > } > if (unlikely(vq->log_used)) { > /* Make sure data is seen before log. */ > - wmb(); > + smp_wmb(); > log_write(vq->log_base, vq->log_addr + sizeof *vq->used->ring * > (vq->last_used_idx % vq->num), > sizeof *vq->used->ring); > @@ -1060,7 +1060,7 @@ bool vhost_enable_notify(struct vhost_virtqueue *vq) > } > /* They could have slipped one in as we were doing that: make > * sure it's written, then check again. */ > - mb(); > + smp_mb(); > r = get_user(avail_idx, &vq->avail->idx); > if (r) { > vq_err(vq, "Failed to check avail idx at %p: %d\n", > -- > 1.6.6.144.g5c3af
On 02/01/2010 07:21 PM, Michael S. Tsirkin wrote:> vhost-net only uses memory barriers to control SMP effects > (communication with userspace potentially running on a different CPU), > so it should use SMP barriers and not mandatory barriers for memory > access ordering, as suggested by Documentation/memory-barriers.txt > >A UP guest running on an SMP host still needs those barriers. -- error compiling committee.c: too many arguments to function
On Sun, Feb 07, 2010 at 11:42:29AM +0200, Avi Kivity wrote:> On 02/01/2010 07:21 PM, Michael S. Tsirkin wrote: >> vhost-net only uses memory barriers to control SMP effects >> (communication with userspace potentially running on a different CPU), >> so it should use SMP barriers and not mandatory barriers for memory >> access ordering, as suggested by Documentation/memory-barriers.txt >> >> > > A UP guest running on an SMP host still needs those barriers.Correct. And since vhost net is running host-side, smp_XX barriers will do exactly the right thing, right?> -- > error compiling committee.c: too many arguments to function
On 02/07/2010 11:44 AM, Michael S. Tsirkin wrote:> On Sun, Feb 07, 2010 at 11:42:29AM +0200, Avi Kivity wrote: > >> On 02/01/2010 07:21 PM, Michael S. Tsirkin wrote: >> >>> vhost-net only uses memory barriers to control SMP effects >>> (communication with userspace potentially running on a different CPU), >>> so it should use SMP barriers and not mandatory barriers for memory >>> access ordering, as suggested by Documentation/memory-barriers.txt >>> >>> >>> >> A UP guest running on an SMP host still needs those barriers. >> > Correct. And since vhost net is running host-side, smp_XX > barriers will do exactly the right thing, right? >Right, of course. Mixed up virtio and vhost. -- error compiling committee.c: too many arguments to function
On Sun, 7 Feb 2010 07:37:49 pm Michael S. Tsirkin wrote:> On Mon, Feb 01, 2010 at 07:21:02PM +0200, Michael S. Tsirkin wrote: > > vhost-net only uses memory barriers to control SMP effects > > (communication with userspace potentially running on a different CPU), > > so it should use SMP barriers and not mandatory barriers for memory > > access ordering, as suggested by Documentation/memory-barriers.txt > > > > Signed-off-by: Michael S. Tsirkin <mst at redhat.com> > > > Rusty, any feedback on this one? > Thanks!Yep. barrier() is correct on UP to guard against preemption. Acked-by: Rusty Russell <rusty at rustcorp.com.au> Thanks, Rusty.