Re: [PATCH] vhost-vsock: fix migration issue when seqpacket is supported

[Daniel P . Berrangé]

On Tue, Sep 07, 2021 at 02:49:35PM +0200, Stefano Garzarella wrote:
> Commit 1e08fd0a46 ("vhost-vsock: SOCK_SEQPACKET feature bit support")
> enabled the SEQPACKET feature bit.
> This commit is released with QEMU 6.1, so if we try to migrate a VM where
> the host kernel supports SEQPACKET but machine type version is less than
> 6.1, we get the following errors:
> 
>     Features 0x130000002 unsupported. Allowed features: 0x179000000
>     Failed to load virtio-vhost_vsock:virtio
>     error while loading state for instance 0x0 of device 
> '0000:00:05.0/virtio-vhost_vsock'
>     load of migration failed: Operation not permitted
> 
> Let's disable the feature bit for machine types < 6.1, adding a
> `features` field to VHostVSock to simplify the handling of upcoming
> features we will support.

IIUC, this will still leave migration broken for anyone migrating
a >= 6.1 machine type between a kernel that supports SEQPACKET and
a kernel lacking that, or vica-verca.  

If a feature is dependant on a host kernel feature we can't turn
that on automatically as part of the machine type, as we need
ABI stability across migration indepdant of kernel version.


Regards,
Daniel
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