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

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Message-ID: <6b3a9a98-c095-1729-3528-dd521f136797@redhat.com>
Date:   Wed, 26 Jul 2017 21:37:15 +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 21:18, Jason Wang wrote:
>
>
> 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

More notes, the previous assumption is that we don't move used event 
back, but this could happen in fact if idx is wrapper around. Will 
repost and add this into commit log.

Thanks