Messages in this thread Patch in this message | | | Date | Tue, 25 Jan 2022 12:14:22 +0100 | From | Stefano Garzarella <> | Subject | Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify() |
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On Mon, Jan 24, 2022 at 11:31:49AM +0000, Stefan Hajnoczi wrote: >On Fri, Jan 14, 2022 at 10:05:08AM +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. >> >> We are not caching the avail index, so when the device will call >> vhost_get_vq_desc(), it will find the old value in the cache and >> it will read the avail index again. > >I think this wording is clearer because we do keep a cached the avail >index value, but the issue is we don't update it: >s/We are not caching the avail index/We do not update the cached avail >index value/
I'll fix in v3. It seems I forgot to CC you on v2: https://lore.kernel.org/virtualization/20220121153108.187291-1-sgarzare@redhat.com/
> >> >> It would be better to refresh the cache every time we read avail >> index, so let's change vhost_enable_notify() caching the value in >> `avail_idx` and compare it with `last_avail_idx` to check if there >> are new buffers available. >> >> Anyway, we don't expect a significant performance boost because >> the above path is not very common, indeed vhost_enable_notify() >> is often called with unlikely(), expecting that avail index has >> not been updated. >> >> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> >> --- >> v1: >> - improved the commit description [MST, Jason] >> --- >> 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; > >vhost_vq_avail_empty() has a fast path that's missing in >vhost_enable_notify(): > > if (vq->avail_idx != vq->last_avail_idx) > return false;
Yep, I thought about that, but devices usually call vhost_enable_notify() right when vq->avail_idx == vq->last_avail_idx, so I don't know if it's an extra check for a branch that will never be taken.
Do you think it is better to add that check? (maybe with unlikely())
Thanks, Stefano
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