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

[Avihai Horon]

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.

Thanks.

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