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

[Michael S. Tsirkin]

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?



> Instead of silent masking of the features virtio-net gracefully
> disables the vhost at set_features if some features acked by the
> guest contradict with kernel vhost capabilities.
> 
> This set of patches also makes get_vhost_net() call (that used
> everywhere) to always return actual result, i.e. initially it
> returns non-NULL value and from the moment the vhost was disabled
> the call will return NULL. Such a way we avoid any unexpected
> calls to vhost functions.
> Yuri Benditovich (3):
>   vhost-net: add VIRTIO_NET_F_HASH_REPORT to the list of kernel features
>   net: add ability to hide (disable) vhost_net
>   virtio-net: graceful fallback to vhost=off for tap netdev
> 
>  hw/net/vhost_net.c  |  5 +++-
>  hw/net/virtio-net.c | 58 ++++++++++++++++++++++++++++++++++++++-------
>  include/net/net.h   |  1 +
>  3 files changed, 55 insertions(+), 9 deletions(-)
> 
> -- 
> 2.17.1