Stefano Garzarella
2022-Jan-13 14:56 UTC
[RFC PATCH] vhost: cache avail index in vhost_enable_notify()
In vhost_enable_notify() we enable the notifications and we read the avail index to check if new buffers have become available in the meantime. In this case, the device would go to re-read avail index to access the descriptor. As we already do in other place, we can cache the value in `avail_idx` and compare it with `last_avail_idx` to check if there are new buffers available. Signed-off-by: Stefano Garzarella <sgarzare at redhat.com> --- drivers/vhost/vhost.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 59edb5a1ffe2..07363dff559e 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) &vq->avail->idx, r); return false; } + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; + return vq->avail_idx != vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_enable_notify); -- 2.31.1
Michael S. Tsirkin
2022-Jan-13 15:19 UTC
[RFC PATCH] vhost: cache avail index in vhost_enable_notify()
On Thu, Jan 13, 2022 at 03:56:42PM +0100, Stefano Garzarella wrote:> In vhost_enable_notify() we enable the notifications and we read > the avail index to check if new buffers have become available in > the meantime. In this case, the device would go to re-read avail > index to access the descriptor. > > As we already do in other place, we can cache the value in `avail_idx` > and compare it with `last_avail_idx` to check if there are new > buffers available. > > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>I guess we can ... but what's the point?> --- > drivers/vhost/vhost.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 59edb5a1ffe2..07363dff559e 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > &vq->avail->idx, r); > return false; > } > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > + return vq->avail_idx != vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_enable_notify); > > -- > 2.31.1
Jason Wang
2022-Jan-14 06:18 UTC
[RFC PATCH] vhost: cache avail index in vhost_enable_notify()
On Thu, Jan 13, 2022 at 10:57 PM Stefano Garzarella <sgarzare at redhat.com> wrote:> > In vhost_enable_notify() we enable the notifications and we read > the avail index to check if new buffers have become available in > the meantime. In this case, the device would go to re-read avail > index to access the descriptor. > > As we already do in other place, we can cache the value in `avail_idx` > and compare it with `last_avail_idx` to check if there are new > buffers available. > > Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>Patch looks fine but I guess we won't get performance improvement since it doesn't save any userspace/VM memory access? Thanks> --- > drivers/vhost/vhost.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index 59edb5a1ffe2..07363dff559e 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > &vq->avail->idx, r); > return false; > } > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > + return vq->avail_idx != vq->last_avail_idx; > } > EXPORT_SYMBOL_GPL(vhost_enable_notify); > > -- > 2.31.1 >