Re: [PATCH 3/3] vfio/migration: Make VFIO migration non-experimental

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

On Tue, 27 Jun 2023 11:00:46 +0300
Avihai Horon <avihaih@nvidia.com> wrote:

> On 26/06/2023 20:27, Alex Williamson wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Mon, 26 Jun 2023 17:26:42 +0200
> > Cédric Le Goater <clg@redhat.com> wrote:
> >  
> >> On 6/26/23 15:40, Joao Martins wrote:  
> >>> On 26/06/2023 14:20, Cédric Le Goater wrote:  
> >>>> Hello Avihai,
> >>>>
> >>>> On 6/26/23 10:23, Avihai Horon wrote:  
> >>>>> The major parts of VFIO migration are supported today in QEMU. This
> >>>>> includes basic VFIO migration, device dirty page tracking and precopy
> >>>>> support.
> >>>>>
> >>>>> Thus, at this point in time, it seems appropriate to make VFIO migration
> >>>>> non-experimental: remove the x prefix from enable_migration property,
> >>>>> change it to ON_OFF_AUTO and let the default value be AUTO.
> >>>>>
> >>>>> In addition, make the following adjustments:
> >>>>> 1. Require device dirty tracking support when enable_migration is AUTO
> >>>>>       (i.e., not explicitly enabled). This is because device dirty tracking
> >>>>>       is currently the only method to do dirty page tracking, which is
> >>>>>       essential for migrating in a reasonable downtime.  
> >>>> hmm, I don't think QEMU should decide to disable a feature for all
> >>>> devices supposedly because it could be slow for some. That's too
> >>>> restrictive. What about devices with have small states ? for which
> >>>> the downtime would be reasonable even without device dirty tracking
> >>>> support.
> >>>>  
> >>> device dirty tracking refers to the ability to tracking dirty IOVA used by the
> >>> device which will DMA into RAM. It is required because the
> >>> consequence/alternative is to transfer all RAM in stop copy phase. Device state
> >>> size at that point is the least of our problems downtime wise.  
> >> Arg. thanks for reminding me. I tend to take this for granted ...  
> > My initial reaction was the same, for instance we could have a non-DMA
> > device (ex. PCI serial card) that supports migration w/o dirty
> > tracking, but QEMU cannot assume the device doesn't do DMA, nor is it
> > worth our time to monitor whether bus-master is ever enabled for the
> > device, so QEMU needs to assume all memory that's DMA accessible for
> > the device is perpetually dirty.  Also, if such a corner case existed
> > for a non-DMA migratable device, the easier path is simply to require
> > dirty tracking stubs in the variant driver to report no memory dirtied.
> >  
> >>> I can imagine that allowing without dirty tracking is useful for developer
> >>> testing of the suspend/device-state flows, but as real default (auto) is very
> >>> questionable to let it go through without dirty tracking. When we have IOMMUFD
> >>> dirty tracking that's when we can relieve this restriction as a default.
> >>>
> >>> But then note that (...)
> >>>  
> >>>>> Setting
> >>>>>       enable_migration to ON will not require device dirty tracking.  
> >>> (...) this lets it ignore dirty tracking as you would like.
> >>>
> >>>  
> >>>>> 2. Make migration blocker messages more elaborate.
> >>>>> 3. Remove error prints in vfio_migration_query_flags().
> >>>>> 4. Remove a redundant assignment in vfio_migration_realize().
> >>>>>
> >>>>> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> >>>>> ---
> >>>>>     include/hw/vfio/vfio-common.h |  2 +-
> >>>>>     hw/vfio/migration.c           | 29 ++++++++++++++++-------------
> >>>>>     hw/vfio/pci.c                 |  4 ++--
> >>>>>     3 files changed, 19 insertions(+), 16 deletions(-)
> >>>>>
> >>>>> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> >>>>> index b4c28f318f..387eabde60 100644
> >>>>> --- a/include/hw/vfio/vfio-common.h
> >>>>> +++ b/include/hw/vfio/vfio-common.h
> >>>>> @@ -139,7 +139,7 @@ typedef struct VFIODevice {
> >>>>>         bool needs_reset;
> >>>>>         bool no_mmap;
> >>>>>         bool ram_block_discard_allowed;
> >>>>> -    bool enable_migration;
> >>>>> +    OnOffAuto enable_migration;
> >>>>>         VFIODeviceOps *ops;
> >>>>>         unsigned int num_irqs;
> >>>>>         unsigned int num_regions;
> >>>>> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> >>>>> index 79eb81dfd7..d8e0848635 100644
> >>>>> --- a/hw/vfio/migration.c
> >>>>> +++ b/hw/vfio/migration.c
> >>>>> @@ -731,14 +731,6 @@ static int vfio_migration_query_flags(VFIODevice
> >>>>> *vbasedev, uint64_t *mig_flags)
> >>>>>         feature->argsz = sizeof(buf);
> >>>>>         feature->flags = VFIO_DEVICE_FEATURE_GET | VFIO_DEVICE_FEATURE_MIGRATION;
> >>>>>         if (ioctl(vbasedev->fd, VFIO_DEVICE_FEATURE, feature)) {
> >>>>> -        if (errno == ENOTTY) {
> >>>>> -            error_report("%s: VFIO migration is not supported in kernel",
> >>>>> -                         vbasedev->name);
> >>>>> -        } else {
> >>>>> -            error_report("%s: Failed to query VFIO migration support, err: %s",
> >>>>> -                         vbasedev->name, strerror(errno));
> >>>>> -        }
> >>>>> -
> >>>>>             return -errno;
> >>>>>         }
> >>>>>     @@ -831,14 +823,28 @@ void vfio_reset_bytes_transferred(void)
> >>>>>       int vfio_migration_realize(VFIODevice *vbasedev, Error **errp)
> >>>>>     {
> >>>>> -    int ret = -ENOTSUP;
> >>>>> +    int ret;
> >>>>>     -    if (!vbasedev->enable_migration) {
> >>>>> +    if (vbasedev->enable_migration == ON_OFF_AUTO_OFF) {
> >>>>> +        error_setg(&vbasedev->migration_blocker,
> >>>>> +                   "%s: Migration is disabled for VFIO device", vbasedev->name);
> >>>>>             goto add_blocker;
> >>>>>         }
> >>>>>           ret = vfio_migration_init(vbasedev);
> >>>>>         if (ret) {  
> >>>> It would be good to keep the message for 'errno == ENOTTY' as it was in
> >>>> vfio_migration_query_flags(). When migration fails, it is an important
> >>>> information to know that it is because the VFIO PCI host device driver
> >>>> doesn't support the feature. The root cause could be deep below in FW or
> >>>> how the VF was set up.
> >>>>  
> >>> +1 As I have been in this rabbit hole  
> 
> Sure, I will add it.
> 
> >>>  
> >>>>> +        error_setg(&vbasedev->migration_blocker,
> >>>>> +                   "%s: Migration couldn't be initialized for VFIO device, "
> >>>>> +                   "err: %d (%s)",
> >>>>> +                   vbasedev->name, ret, strerror(-ret));
> >>>>> +        goto add_blocker;
> >>>>> +    }
> >>>>> +
> >>>>> +    if (vbasedev->enable_migration == ON_OFF_AUTO_AUTO &&
> >>>>> +        !vbasedev->dirty_pages_supported) {  
> >>>> I don't agree with this test.
> >>>>  
> >>> The alternative right now is perceptual dirty tracking. How is that OK as a
> >>> default? It's not like we have even an option :(
> >>>
> >>> Maybe perhaps you refer to avoid strongly enforcing *always* it to allow testing
> >>> of the non dirty tracking parts? Maybe when you 'force' enabling with
> >>> enable-migration=on is when you ignore the dirty tracking which is what this is
> >>> doing.  
> >>
> >> I see ON_OFF_AUTO_ON as a way to abort the machine startup while
> >> ON_OFF_AUTO_AUTO would let it run but block migration.  
> > Agreed.  There's a little bit of redundancy between the device-level
> > "enable-migration=on" option and the global "-only-migratable" option
> > relative to preventing machine startup, but it also doesn't make sense
> > to me if the device-level option let realize complete successfully if
> > the device doesn't support or fails migration setup.  So I think we'd
> > generally rely on using the -only-migratable option with the default
> > ON_OFF_AUTO_AUTO value, allow the ON_OFF_AUTO_ON value to enable
> > dis-recommended support, and live with the redundancy that it should
> > also cause the device realize to fail if migration is not supported.
> > Thanks,
> >
> > Alex  
> 
> OK.
> 
> When enable_migration=AUTO we allow blockers.
> When enable_migration=ON we don't allow blockers and instead prevent 
> realization of VFIO device.
> 
> Regarding device dirty tracking, we keep current behavior, right?
> That is:
> When enable_migration=AUTO we block migration if device dirty tracking 
> is not supported.
> When enable_migration=ON we allow migration even if device dirty 
> tracking is not supported (in which case DMA-able memory will be 
> perpetually dirtied).

Yes, and I think we'd be justified in logging a warning when migration
is enabled without any dirty page tracking support.  Thanks,

Alex