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

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

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?

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?  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),