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
