netdev - Re: [PATCH net] Revert "vhost: cache used event for better performance"

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Message-ID: <117542fc-eef9-c043-7c9e-daafceb7db4e@redhat.com>
Date:   Wed, 26 Jul 2017 21:18:02 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] Revert "vhost: cache used event for better
 performance"



On 2017年07月26日 20:57, Michael S. Tsirkin wrote:
> On Wed, Jul 26, 2017 at 04:03:17PM +0800, Jason Wang wrote:
>> This reverts commit 809ecb9bca6a9424ccd392d67e368160f8b76c92. Since it
>> was reported to break vhost_net. We want to cache used event and use
>> it to check for notification. We try to valid cached used event by
>> checking whether or not it was ahead of new, but this is not correct
>> all the time, it could be stale and there's no way to know about this.
>>
>> Signed-off-by: Jason Wang<jasowang@...hat.com>
> Could you supply a bit more data here please?  How does it get stale?
> What does guest need to do to make it stale?  This will be helpful if
> anyone wants to bring it back, or if we want to extend the protocol.
>

The problem we don't know whether or not guest has published a new used 
event. The check vring_need_event(vq->last_used_event, new + vq->num, 
new) is not sufficient to check for this.

Thanks