Re: [PATCH v1 1/1] vfio: Make migration support non experimental by default.

[Shenming Lu <lushenming@huawei.com>]

On 2021/3/9 6:51, Alex Williamson wrote:
> [Cc +Intel]
> 
> On Mon, 8 Mar 2021 21:39:49 +0530
> Tarun Gupta <targupta@nvidia.com> wrote:
> 
>> VFIO migration support in QEMU is experimental as of now, which was done to
>> provide soak time and resolve concerns regarding bit-stream.
>> But, with the patches discussed in
>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg784931.html , we have
>> corrected ordering of saving PCI config space and bit-stream.
>>
>> So, this patch proposes to make vfio migration support in QEMU to be enabled
>> by default. Tested by successfully migrating mdev device.
>>
>> Signed-off-by: Tarun Gupta <targupta@nvidia.com>
>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>> ---
>>  hw/vfio/pci.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
>> index f74be78209..15e26f460b 100644
>> --- a/hw/vfio/pci.c
>> +++ b/hw/vfio/pci.c
>> @@ -3199,7 +3199,7 @@ static Property vfio_pci_dev_properties[] = {
>>      DEFINE_PROP_BIT("x-igd-opregion", VFIOPCIDevice, features,
>>                      VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false),
>>      DEFINE_PROP_BOOL("x-enable-migration", VFIOPCIDevice,
>> -                     vbasedev.enable_migration, false),
>> +                     vbasedev.enable_migration, true),
>>      DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),
>>      DEFINE_PROP_BOOL("x-balloon-allowed", VFIOPCIDevice,
>>                       vbasedev.ram_block_discard_allowed, false),
> 
> Looking back at the commit where this was added:
> 
> commit cf254988a50d4164c86a356c80b8d3ae0ccaa005
> Author: Alex Williamson <alex.williamson@redhat.com>
> Date:   Mon Nov 9 11:56:02 2020 -0700
> 
>     vfio: Make migration support experimental
>     
>     Support for migration of vfio devices is still in flux.  Developers
>     are attempting to add support for new devices and new architectures,
>     but none are yet readily available for validation.  We have concerns
>     whether we're transferring device resources at the right point in the
>     migration, whether we're guaranteeing that updates during pre-copy are
>     migrated, and whether we can provide bit-stream compatibility should
>     any of this change.  Even the question of whether devices should
>     participate in dirty page tracking during pre-copy seems contentious.
>     In short, migration support has not had enough soak time and it feels
>     premature to mark it as supported.
>     
>     Create an experimental option such that we can continue to develop.
>     
>     [Retaining previous acks/reviews for a previously identical code
>      change with different specifics in the commit log.]
>     
>     Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>     Acked-by: Cornelia Huck <cohuck@redhat.com>
>     Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> 
> 
> What has tangibly changed since then?  I think we have patches on-list
> to address the known issue of PCI config space (MSI) ordering, which
> related to enabling migration on ARM platforms.  Do we have
> significantly more confidence in our ability to make compatible
> enhancement to the migration bitstream?  This was a particularly
> troublesome point for me if we have any hope of calling this
> supportable.  As far as I know, there are still no open source vendor
> drivers supporting migration for community testing.  We're also still
> missing the documentation that was promised previously, as Connie noted.
> 
> Huawei and Intel, what's your confidence level and what can you share
> regarding support for this implementation?  Thanks,

We have sent a number of patches regarding VFIO migration from our own test
(the support for this in our accelerator driver is still in experiment),
some of them are still on-list []...

[] https://lore.kernel.org/qemu-devel/20210310094106.2191-2-jiangkunkun@huawei.com/
   https://lore.kernel.org/linux-arm-kernel/20210126124444.27136-1-zhukeqian1@huawei.com/

Thanks,
Shenming

> 
> Alex
> 
> .
>