Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

[Stefano Garzarella]

On Fri, Jan 14, 2022 at 07:45:35AM -0500, Michael S. Tsirkin 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.
> >
> > 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@xxxxxxxxxx>
>
> ... and can in theory even hurt due to an extra memory write.
> So ... performance test restults pls?

Right, could be.

I'll run some perf test with vsock, about net, do you have a test suite 
or common step to follow to test it?

Thanks,
Stefano