Re: [PATCH v6 32/36] vfio/migration: Make x-migration-multifd-transfer VFIO property mutable

["Cédric Le Goater" <clg@redhat.com>]

On 3/4/25 23:03, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> DEFINE_PROP_ON_OFF_AUTO() property isn't runtime-mutable so using it
> would mean that the source VM would need to decide upfront at startup
> time whether it wants to do a multifd device state transfer at some
> point.
> 
> Source VM can run for a long time before being migrated so it is
> desirable to have a fallback mechanism to the old way of transferring
> VFIO device state if it turns to be necessary.
> 
> This brings this property to the same mutability level as ordinary
> migration parameters, which too can be adjusted at the run time.
> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>


Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks,

C.


> ---
>   hw/vfio/migration-multifd.c |  4 ++++
>   hw/vfio/pci.c               | 20 +++++++++++++++++---
>   2 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/vfio/migration-multifd.c b/hw/vfio/migration-multifd.c
> index aacddc503bb8..233724710b37 100644
> --- a/hw/vfio/migration-multifd.c
> +++ b/hw/vfio/migration-multifd.c
> @@ -485,6 +485,10 @@ bool vfio_multifd_setup(VFIODevice *vbasedev, bool alloc_multifd, Error **errp)
>   {
>       VFIOMigration *migration = vbasedev->migration;
>   
> +    /*
> +     * Make a copy of this setting at the start in case it is changed
> +     * mid-migration.
> +     */
>       if (vbasedev->migration_multifd_transfer == ON_OFF_AUTO_AUTO) {
>           migration->multifd_transfer = vfio_multifd_transfer_supported();
>       } else {
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 6d6f889f59c5..21605bac2fb0 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -3353,6 +3353,8 @@ static void vfio_instance_init(Object *obj)
>       pci_dev->cap_present |= QEMU_PCI_CAP_EXPRESS;
>   }
>   
> +static PropertyInfo vfio_pci_migration_multifd_transfer_prop;
> +
>   static const Property vfio_pci_dev_properties[] = {
>       DEFINE_PROP_PCI_HOST_DEVADDR("host", VFIOPCIDevice, host),
>       DEFINE_PROP_UUID_NODEFAULT("vf-token", VFIOPCIDevice, vf_token),
> @@ -3377,9 +3379,10 @@ static const Property vfio_pci_dev_properties[] = {
>                       VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false),
>       DEFINE_PROP_ON_OFF_AUTO("enable-migration", VFIOPCIDevice,
>                               vbasedev.enable_migration, ON_OFF_AUTO_AUTO),
> -    DEFINE_PROP_ON_OFF_AUTO("x-migration-multifd-transfer", VFIOPCIDevice,
> -                            vbasedev.migration_multifd_transfer,
> -                            ON_OFF_AUTO_AUTO),
> +    DEFINE_PROP("x-migration-multifd-transfer", VFIOPCIDevice,
> +                vbasedev.migration_multifd_transfer,
> +                vfio_pci_migration_multifd_transfer_prop, OnOffAuto,
> +                .set_default = true, .defval.i = ON_OFF_AUTO_AUTO),
>       DEFINE_PROP_BOOL("migration-events", VFIOPCIDevice,
>                        vbasedev.migration_events, false),
>       DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),
> @@ -3480,6 +3483,17 @@ static const TypeInfo vfio_pci_nohotplug_dev_info = {
>   
>   static void register_vfio_pci_dev_type(void)
>   {
> +    /*
> +     * Ordinary ON_OFF_AUTO property isn't runtime-mutable, but source VM can
> +     * run for a long time before being migrated so it is desirable to have a
> +     * fallback mechanism to the old way of transferring VFIO device state if
> +     * it turns to be necessary.
> +     * The following makes this type of property have the same mutability level
> +     * as ordinary migration parameters.
> +     */
> +    vfio_pci_migration_multifd_transfer_prop = qdev_prop_on_off_auto;
> +    vfio_pci_migration_multifd_transfer_prop.realized_set_allowed = true;
> +
>       type_register_static(&vfio_pci_dev_info);
>       type_register_static(&vfio_pci_nohotplug_dev_info);
>   }
>