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:

quoted

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@redhat.com>

... 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