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Re: [PATCH 3/3] virtio-net: graceful fallback to vhost=off for tap netde

From: Jason Wang
Subject: Re: [PATCH 3/3] virtio-net: graceful fallback to vhost=off for tap netdev
Date: Tue, 9 Feb 2021 11:39:36 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2021/2/9 上午3:46, Yuri Benditovich wrote:
On Mon, Feb 8, 2021 at 5:15 AM Jason Wang <jasowang@redhat.com> wrote:

On 2021/2/5 下午9:38, Michael S. Tsirkin wrote:
On Thu, Feb 04, 2021 at 10:29:15PM +0200, Yuri Benditovich wrote:
Currently virtio-net silently clears features if they are
not supported by respective vhost. This may create migration
problems in future if vhost features on the source and destination
are different. Implement graceful fallback to no-vhost mode
when some acked features contradict with vhost. The decision is
taken on set_features call and the vhost will be disabled
till next reset (or migration).
Such fallback is currently enabled only for TAP netdev.

Signed-off-by: Yuri Benditovich<yuri.benditovich@daynix.com>
Sounds good, but I don't think we should do this if
vhostforce=on is set.

If we do this, does it mean we won't maintain migration compatibility
when vhostforce is on?
AFAIU, the 'vhostforce=on' should mean the vhost can't be disabled (if
I'm not mistaken this is typically used for vhost-user).
So we can view this case as similar to vhost-vdpa and vhost-user.

Right, but since it was used by libivrt. Then it turns out to be a compatibility breaker.



Also, let's document this behaviour with the vhost option so people
are not suprized.

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