netdev - Re: [PATCH v1] vhost: cache avail index in vhost_enable

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Date:   Fri, 14 Jan 2022 14:38:16 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, stefanha@...hat.com,
        kvm@...r.kernel.org, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

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@...hat.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