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

[Stefano Garzarella]

On Fri, Jan 14, 2022 at 02:18:01PM +0800, Jason Wang wrote:
> On Thu, Jan 13, 2022 at 10:57 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> Patch looks fine but I guess we won't get performance improvement
> since it doesn't save any userspace/VM memory access?

It should save the memory access when vhost_enable_notify() find 
something new in the VQ, so in this path:

    vhost_enable_notify() <- VM memory access for avail index
      == true

    vhost_disable_notify()
    ...

    vhost_get_vq_desc()   <- VM memory access for avail index
                             with the patch applied, this access is 
                             avoided since avail index is cached

In any case, I don't expect this to be a very common path, indeed we
usually use unlikely() for this path:

    if (unlikely(vhost_enable_notify(dev, vq))) {
        vhost_disable_notify(dev, vq);
        continue;
    }

So I don't expect a significant performance increase.

v1 coming with a better commit description.

Thanks,
Stefano