* Alex Williamson ([email protected]) wrote: > Per the proposed documentation for vfio device migration: > > Dirty pages are tracked when device is in stop-and-copy phase > because if pages are marked dirty during pre-copy phase and > content is transfered from source to destination, there is no > way to know newly dirtied pages from the point they were copied > earlier until device stops. To avoid repeated copy of same > content, pinned pages are marked dirty only during > stop-and-copy phase. > > Essentially, since we don't have hardware dirty page tracking for > assigned devices at this point, we consider any page that is pinned > by an mdev vendor driver or pinned and mapped through the IOMMU to > be perpetually dirty. In the worst case, this may result in all of > guest memory being considered dirty during every iteration of live > migration. The current vfio implementation of migration has chosen > to mask device dirtied pages until the final stages of migration in > order to avoid this worst case scenario. > > Allowing the device to implement a policy decision to prioritize > reduced migration data like this jeopardizes QEMU's overall ability > to implement any degree of service level guarantees during migration. > For example, any estimates towards achieving acceptable downtime > margins cannot be trusted when such a device is present. The vfio > device should participate in dirty page tracking to the best of its > ability throughout migration, even if that means the dirty footprint > of the device impedes migration progress, allowing both QEMU and > higher level management tools to decide whether to continue the > migration or abort due to failure to achieve the desired behavior.
I don't feel particularly badly about the decision to squash it in during the stop-and-copy phase; for devices where the pinned memory is large, I don't think doing it during the main phase makes much sense; especially if you then have to deal with tracking changes in pinning. Having said that, I agree with marking it as experimental, because I'm dubious how useful it will be for the same reason, I worry about whether the downtime will be so large to make it pointless. Reviewed-by: Dr. David Alan Gilbert <[email protected]> > Link: https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg00807.html > Cc: Kirti Wankhede <[email protected]> > Cc: Neo Jia <[email protected]> > Cc: Dr. David Alan Gilbert <[email protected]> > Cc: Juan Quintela <[email protected]> > Cc: Philippe Mathieu-Daudé <[email protected]> > Cc: Cornelia Huck <[email protected]> > Signed-off-by: Alex Williamson <[email protected]> > --- > > Given that our discussion in the link above seems to be going in > circles, I'm afraid it seems necessary to both have a contigency > plan and to raise the visibility of the current behavior to > determine whether others agree that this is a sufficiently > troubling behavior to consider migration support experimental > at this stage. Please voice your opinion or contribute patches > to resolve this before QEMU 5.2. Thanks, > > Alex > > hw/vfio/migration.c | 2 +- > hw/vfio/pci.c | 2 ++ > include/hw/vfio/vfio-common.h | 1 + > 3 files changed, 4 insertions(+), 1 deletion(-) > > diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c > index 3ce285ea395d..cd44d465a50b 100644 > --- a/hw/vfio/migration.c > +++ b/hw/vfio/migration.c > @@ -882,7 +882,7 @@ int vfio_migration_probe(VFIODevice *vbasedev, Error > **errp) > Error *local_err = NULL; > int ret = -ENOTSUP; > > - if (!container->dirty_pages_supported) { > + if (!vbasedev->enable_migration || !container->dirty_pages_supported) { > goto add_blocker; > } > > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c > index 58c0ce8971e3..1349b900e513 100644 > --- a/hw/vfio/pci.c > +++ b/hw/vfio/pci.c > @@ -3194,6 +3194,8 @@ static Property vfio_pci_dev_properties[] = { > VFIO_FEATURE_ENABLE_REQ_BIT, true), > DEFINE_PROP_BIT("x-igd-opregion", VFIOPCIDevice, features, > VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false), > + DEFINE_PROP_BOOL("x-enable-migration", VFIOPCIDevice, > + vbasedev.enable_migration, false), > DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false), > DEFINE_PROP_BOOL("x-balloon-allowed", VFIOPCIDevice, > vbasedev.ram_block_discard_allowed, false), > diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h > index baeb4dcff102..2119872c8af1 100644 > --- a/include/hw/vfio/vfio-common.h > +++ b/include/hw/vfio/vfio-common.h > @@ -123,6 +123,7 @@ typedef struct VFIODevice { > bool needs_reset; > bool no_mmap; > bool ram_block_discard_allowed; > + bool enable_migration; > VFIODeviceOps *ops; > unsigned int num_irqs; > unsigned int num_regions; > -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
