Re: [PATCH v10 03/12] vfio/migration: Allow migration without VFIO IOMMU dirty tracking support

[Juan Quintela]

Avihai Horon <avihaih@nvidia.com> wrote:
> On 15/02/2023 14:43, Juan Quintela wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> Avihai Horon <avihaih@nvidia.com> wrote:
>>> Currently, if IOMMU of a VFIO container doesn't support dirty page
>>> tracking, migration is blocked. This is because a DMA-able VFIO device
>>> can dirty RAM pages without updating QEMU about it, thus breaking the
>>> migration.
>>>
>>> However, this doesn't mean that migration can't be done at all.
>>> In such case, allow migration and let QEMU VFIO code mark all pages
>>> dirty.
>>>
>>> This guarantees that all pages that might have gotten dirty are reported
>>> back, and thus guarantees a valid migration even without VFIO IOMMU
>>> dirty tracking support.
>>>
>>> The motivation for this patch is the introduction of iommufd [1].
>>> iommufd can directly implement the /dev/vfio/vfio container IOCTLs by
>>> mapping them into its internal ops, allowing the usage of these IOCTLs
>>> over iommufd. However, VFIO IOMMU dirty tracking is not supported by
>>> this VFIO compatibility API.
>>>
>>> This patch will allow migration by hosts that use the VFIO compatibility
>>> API and prevent migration regressions caused by the lack of VFIO IOMMU
>>> dirty tracking support.
>>>
>>> [1]
>>> https://lore.kernel.org/kvm/0-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com/
>>>
>>> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
>>> Reviewed-by: Cédric Le Goater <clg@redhat.com>
>> Reviewed-by: Juan Quintela <quintela@redhat.com>
>>
>> I know why you are doing this.
>>
>> But I think this should print a warning, error, somewhere.
>
> IMHO, I'm not sure it's really necessary.
>
> To enable VFIO migration the user must explicitly set x-enable-migration=on.
> I guess in this case the user is well aware of the dirty tracking
> capabilities the VFIO device has or doesn't have.
> So I don't think adding this error/warning will help much.

Oops.  I missed that bit.
I retire my objection.

Sorry, Juan.

> Thanks.
>
>>
>> You are just dirtying all pages each time we arrive here.
>>
>> Even calling the featura "experimental" is an understatement.
>>
>> Later, Juan.
>>