Re: [PATCH 0/4] vhost/vsock: add support for VHOST_RESET_OWNER and CPR migration

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

Hello Stefano,

On 6/16/26 4:35 PM, Stefano Garzarella wrote:
> Hi Andrey,
> thanks for the series!
> 
> On Fri, Jun 12, 2026 at 07:57:14PM +0300, Andrey Drobyshev wrote:
>> Host<-->guest connections via AF_VSOCK sockets aren't supposed to
>> outlive VM migration, since VM is moving to another host.  However
>> there's a special case, which is QEMU live-update, or CPR
>> (checkpoint-restore) migration.  In this case, VM remains on the same
>> host, and we'd like such connections to persist.
> 
> In the spec we have VIRTIO_VSOCK_EVENT_TRANSPORT_RESET which is usually 
> sent by the device after a migration.
> 
> IIUC the specs don't say this has to be done all the time, so we don't 
> need to change anything in the specs, right?
> 
> We just need to avoid sending it (which I think is what we're doing 
> here... I still need to look at the patches).
>

Sending this exact ioctl is guarded by one of my patches in the QEMU
counterpart series:

https://lore.kernel.org/qemu-devel/20260612165110.431376-6-andrey.drobyshev@virtuozzo.com/

So we indeed avoid sending it on migration target in case of CPR migration.

>>
>> For this to work, we need to be able to transfer device ownership from
>> source QEMU to dest QEMU.  Namely, source needs to reset ownership by
>> issuing VHOST_RESET_OWNER ioctl, and then target has to claim it by
>> calling VHOST_SET_OWNER.
>>
>> Since VHOST_RESET_OWNER isn't yet implemented for vhost-vsock, let's add
>> such implementation (patches 1-2).  Also fix regression introduced by
>> the earlier commit [1] (patch 3), and fix the deadlock bug (commit 4).
> 
> If it's a regression, should we fix it separately?
> 
> Or is it related to this series?
>

Probably my wording wasn't quite correct.  I posted this patch here
because we found the problem during testing this particular
functionality, i.e. vsock data transfer + CPR migration.  And the
problem was introduced by a recent commit, which is fine on its own, but
breaks the CPR case.
>>
>> There's a complementary series for QEMU [0] adding support of vhost-vsock
>> devices during CPR migration.
>>
>> NOTE: this series needs to be applied on top of Michael's vhost/linux-next
>> tree as it contains relevant commit [1], not yet present in master branch.
>>
>> I've tested this (patched QEMU + patched kernel) approximately as follows:
>>
>>  * Run listener in the guest:
>>  socat -u VSOCK-LISTEN:9999 - >/tmp/recv.bin
>>
>>  * Run data transfer from host to guest:
>>  socat -u FILE:/root/bigfile.bin VSOCK-CONNECT:CID:9999
>>
>>  * Perform CPR migration during transfer (either cpr-exec or cpr-transfer)
>>  * Check that file hash sum matches
>>
>> [0] https://lore.kernel.org/qemu-devel/20260612165110.431376-1-andrey.drobyshev@virtuozzo.com
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/commit/?id=bb26ed5f3a8b
>>
>> Andrey Drobyshev (1):
>>  vhost/vsock: re-scan TX virtqueue on device start
>>
>> Denis V. Lunev (1):
>>  vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause
>>
>> Pavel Tikhomirov (2):
>>  vhost/vsock: split out vhost_vsock_drop_backends helper
>>  vhost/vsock: add VHOST_RESET_OWNER ioctl
>>
>> drivers/vhost/vsock.c | 80 +++++++++++++++++++++++++++++++++++++------
>> 1 file changed, 69 insertions(+), 11 deletions(-)
>>
>> -- 
>> 2.47.1
>>
>