Re: [PATCH v5 8/9] vfio/migration: Add x-allow-pre-copy VFIO device property

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 4 Jun 2023 12:33:43 +0300
Avihai Horon <avihaih@nvidia.com> wrote:

> On 01/06/2023 23:22, Alex Williamson wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Tue, 30 May 2023 17:48:20 +0300
> > Avihai Horon <avihaih@nvidia.com> wrote:
> >  
> >> Add a new VFIO device property x-allow-pre-copy to keep migration
> >> compatibility to/from older QEMU versions that don't have VFIO pre-copy
> >> support.  
> > This doesn't make sense to me, vfio migration is not currently
> > supported, it can only be enabled via an experimental flag.  AFAIK we
> > have no obligation to maintain migration compatibility against
> > experimental features.  Is there any other reason we need a flag to
> > disable pre-copy?  
> 
> This could give flexibility to do migration between hosts without 
> matching VFIO device kernel drivers. E.g., source driver doesn't have 
> precopy support and dest driver has or vice versa.

If these are valid scenarios, the protocol should support negotiation
without requiring an experimental flag to do so.

> > OTOH, should this series finally remove the experimental migration
> > flag?  Do we require Joao's vIOMMU support to finally make it
> > supportable?  Is there something else?  
> 
> I think that after precopy is accepted we can remove the experimental 
> flag, as we'll have the major parts of VFIO migration upstream.
> After that we will still need to add Joao's vIOMMU support and P2P support.
> Do you want me to add a patch to this series that makes VFIO migration 
> non-experimental?

I'd keep it as a separate patch with a clearly described dependency on
this series so that we can discuss it separately.  Thanks,

Alex

> >> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> >> Reviewed-by: Cédric Le Goater <clg@redhat.com>
> >> ---
> >>   include/hw/vfio/vfio-common.h | 1 +
> >>   hw/core/machine.c             | 1 +
> >>   hw/vfio/migration.c           | 3 ++-
> >>   hw/vfio/pci.c                 | 2 ++
> >>   4 files changed, 6 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> >> index 1db901c194..a53ecbe2e0 100644
> >> --- a/include/hw/vfio/vfio-common.h
> >> +++ b/include/hw/vfio/vfio-common.h
> >> @@ -146,6 +146,7 @@ typedef struct VFIODevice {
> >>       VFIOMigration *migration;
> >>       Error *migration_blocker;
> >>       OnOffAuto pre_copy_dirty_page_tracking;
> >> +    bool allow_pre_copy;
> >>       bool dirty_pages_supported;
> >>       bool dirty_tracking;
> >>   } VFIODevice;
> >> diff --git a/hw/core/machine.c b/hw/core/machine.c
> >> index 1000406211..64ac3fe38e 100644
> >> --- a/hw/core/machine.c
> >> +++ b/hw/core/machine.c
> >> @@ -41,6 +41,7 @@
> >>
> >>   GlobalProperty hw_compat_8_0[] = {
> >>       { "migration", "multifd-flush-after-each-section", "on"},
> >> +    { "vfio-pci", "x-allow-pre-copy", "false" },
> >>   };
> >>   const size_t hw_compat_8_0_len = G_N_ELEMENTS(hw_compat_8_0);
> >>
> >> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> >> index d8f6a22ae1..cb6923ed3f 100644
> >> --- a/hw/vfio/migration.c
> >> +++ b/hw/vfio/migration.c
> >> @@ -323,7 +323,8 @@ static bool vfio_precopy_supported(VFIODevice *vbasedev)
> >>   {
> >>       VFIOMigration *migration = vbasedev->migration;
> >>
> >> -    return migration->mig_flags & VFIO_MIGRATION_PRE_COPY;
> >> +    return vbasedev->allow_pre_copy &&
> >> +           migration->mig_flags & VFIO_MIGRATION_PRE_COPY;
> >>   }
> >>
> >>   /* ---------------------------------------------------------------------- */
> >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >> index 73874a94de..c69813af7f 100644
> >> --- a/hw/vfio/pci.c
> >> +++ b/hw/vfio/pci.c
> >> @@ -3335,6 +3335,8 @@ static Property vfio_pci_dev_properties[] = {
> >>       DEFINE_PROP_ON_OFF_AUTO("x-pre-copy-dirty-page-tracking", VFIOPCIDevice,
> >>                               vbasedev.pre_copy_dirty_page_tracking,
> >>                               ON_OFF_AUTO_ON),
> >> +    DEFINE_PROP_BOOL("x-allow-pre-copy", VFIOPCIDevice,
> >> +                     vbasedev.allow_pre_copy, true),
> >>       DEFINE_PROP_ON_OFF_AUTO("display", VFIOPCIDevice,
> >>                               display, ON_OFF_AUTO_OFF),
> >>       DEFINE_PROP_UINT32("xres", VFIOPCIDevice, display_xres, 0),  
>