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Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP

From: Daniel P . Berrangé
Subject: Re: [PATCH 0/3] virtio-net: graceful drop of vhost for TAP
Date: Tue, 9 Feb 2021 14:51:05 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

On Tue, Feb 09, 2021 at 09:34:20AM -0500, Michael S. Tsirkin wrote:
> On Thu, Feb 04, 2021 at 10:29:12PM +0200, Yuri Benditovich wrote:
> > This set of patches introduces graceful switch from tap-vhost to
> > tap-no-vhost depending on guest features. Before that the features
> > that vhost does not support were silently cleared in get_features.
> > This creates potential problem of migration from the machine where
> > some of virtio-net features are supported by the vhost kernel to the
> > machine where they are not supported (packed ring as an example).
> I still worry that adding new features will silently disable vhost for people.
> Can we limit the change to when a VM is migrated in?

Some management applications expect bi-directional live migration to
work, so taking specific actions on incoming migration only feels

IMHO if the features we're adding cannot be expected to exist in
host kernels in general, then the feature should defualt to off
and require explicit user config to enable.

Downstream distros which can guarantee newer kernels can flip the
default in their custom machine types if they desire.

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