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

[Stefano Garzarella]

On Tue, Sep 07, 2021 at 02:22:24PM +0100, Daniel P. Berrangé wrote:
> 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.

This should be true for migrating from kernel that supports SEQPACKET to 
a kernel lacking that.

For vice-versa I'm not sure, since vhost_get_features() will disable 
that feature if the host kernel doesn't support it, and the guest will 
not have acked it.

> 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.

How do we typically handle this?

I wrongly thought it was an expected behavior that migrating a guest 
using a vhost device from a new kernel to an old one can fail if not all 
features are supported.

I need to take a look at the other vhost devices.

Thanks,
Stefano