Re: [PATCH v2 4/5] vfio/migration: Block VFIO migration with postcopy migration

[Peter Xu <peterx@redhat.com>]

On Thu, Aug 31, 2023 at 03:57:01PM +0300, Avihai Horon wrote:
> VFIO migration is not compatible with postcopy migration. A VFIO device
> in the destination can't handle page faults for pages that have not been
> sent yet.
> 
> Doing such migration will cause the VM to crash in the destination:
> 
> qemu-system-x86_64: VFIO_MAP_DMA failed: Bad address
> qemu-system-x86_64: vfio_dma_map(0x55a28c7659d0, 0xc0000, 0xb000, 0x7f1b11a00000) = -14 (Bad address)
> qemu: hardware error: vfio: DMA mapping failed, unable to continue
> 
> To prevent this, block VFIO migration with postcopy migration.
> 
> Reported-by: Yanghang Liu <yanghliu@redhat.com>
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> ---
>  hw/vfio/migration.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 71855468fe..20994dc1d6 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -335,6 +335,27 @@ static bool vfio_precopy_supported(VFIODevice *vbasedev)
>  
>  /* ---------------------------------------------------------------------- */
>  
> +static int vfio_save_prepare(void *opaque, Error **errp)
> +{
> +    VFIODevice *vbasedev = opaque;
> +
> +    /*
> +     * Snapshot doesn't use postcopy, so allow snapshot even if postcopy is on.
> +     */
> +    if (runstate_check(RUN_STATE_SAVE_VM)) {
> +        return 0;
> +    }

Just purely curious: will it really work to save a snapshot for the GPU
assigned use case?

> +
> +    if (migrate_postcopy_ram()) {
> +        error_setg(
> +            errp, "%s: VFIO migration is not supported with postcopy migration",
> +            vbasedev->name);
> +        return -EOPNOTSUPP;
> +    }
> +
> +    return 0;
> +}

-- 
Peter Xu