Date:   Fri, 14 Jan 2022 14:38:16 +0100
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, stefanha@redhat.com,
        kvm@vger.kernel.org, netdev@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()
Message-ID: <20220114133816.7niyaqygvdveddmi@steredhat>
References: <20220114090508.36416-1-sgarzare@redhat.com>
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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@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