Re: [PATCH Kernel v22 0/8] Add UAPIs to support migration for VFIO devices

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Yan Zhao (yan.y.zhao@intel.com) wrote:
> On Tue, May 26, 2020 at 02:19:39PM -0600, Alex Williamson wrote:
> > On Mon, 25 May 2020 18:50:54 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> > 
> > > On 5/25/2020 12:29 PM, Yan Zhao wrote:
> > > > On Tue, May 19, 2020 at 10:58:04AM -0600, Alex Williamson wrote:  
> > > >> Hi folks,
> > > >>
> > > >> My impression is that we're getting pretty close to a workable
> > > >> implementation here with v22 plus respins of patches 5, 6, and 8.  We
> > > >> also have a matching QEMU series and a proposal for a new i40e
> > > >> consumer, as well as I assume GVT-g updates happening internally at
> > > >> Intel.  I expect all of the latter needs further review and discussion,
> > > >> but we should be at the point where we can validate these proposed
> > > >> kernel interfaces.  Therefore I'd like to make a call for reviews so
> > > >> that we can get this wrapped up for the v5.8 merge window.  I know
> > > >> Connie has some outstanding documentation comments and I'd like to make
> > > >> sure everyone has an opportunity to check that their comments have been
> > > >> addressed and we don't discover any new blocking issues.  Please send
> > > >> your Acked-by/Reviewed-by/Tested-by tags if you're satisfied with this
> > > >> interface and implementation.  Thanks!
> > > >>  
> > > > hi Alex
> > > > after porting gvt/i40e vf migration code to kernel/qemu v23, we spoted
> > > > two bugs.
> > > > 1. "Failed to get dirty bitmap for iova: 0xfe011000 size: 0x3fb0 err: 22"
> > > >     This is a qemu bug that the dirty bitmap query range is not the same
> > > >     as the dma map range. It can be fixed in qemu. and I just have a little
> > > >     concern for kernel to have this restriction.
> > > >   
> > > 
> > > I never saw this unaligned size in my testing. In this case if you can 
> > > provide vfio_* event traces, that will helpful.
> > 
> > Yeah, I'm curious why we're hitting such a call path, I think we were
> > designing this under the assumption we wouldn't see these.  I also
> that's because the algorithm for getting dirty bitmap query range is still not exactly
> matching to that for dma map range in vfio_dma_map().
> 
> 
> > wonder if we really need to enforce the dma mapping range for getting
> > the dirty bitmap with the current implementation (unmap+dirty obviously
> > still has the restriction).  We do shift the bitmap in place for
> > alignment, but I'm not sure why we couldn't shift it back and only
> > clear the range that was reported.  Kirti, do you see other issues?  I
> > think a patch to lift that restriction is something we could plan to
> > include after the initial series is included and before we've committed
> > to the uapi at the v5.8 release.
> >  
> > > > 2. migration abortion, reporting
> > > > "qemu-system-x86_64-lm: vfio_load_state: Error allocating buffer
> > > > qemu-system-x86_64-lm: error while loading state section id 49(vfio)
> > > > qemu-system-x86_64-lm: load of migration failed: Cannot allocate memory"
> > > > 
> > > > It's still a qemu bug and we can fixed it by
> > > > "
> > > > if (migration->pending_bytes == 0) {
> > > > +            qemu_put_be64(f, 0);
> > > > +            qemu_put_be64(f, VFIO_MIG_FLAG_END_OF_STATE);
> > > > "  
> > > 
> > > In which function in QEMU do you have to add this?
> > 
> > I think this is relative to QEMU path 09/ where Yan had the questions
> > below on v16 and again tried to get answers to them on v22:
> > 
> > https://lore.kernel.org/qemu-devel/20200520031323.GB10369@joy-OptiPlex-7040/
> > 
> > Kirti, please address these questions.
> > 
> > > > and actually there are some extra concerns about this part, as reported in
> > > > [1][2].
> > > > 
> > > > [1] data_size should be read ahead of data_offset
> > > > https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg02795.html.
> > > > [2] should not repeatedly update pending_bytes in vfio_save_iterate()
> > > > https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg02796.html.
> > > > 
> > > > but as those errors are all in qemu, and we have finished basic tests in
> > > > both gvt & i40e, we're fine with the kernel part interface in general now.
> > > > (except for my concern [1], which needs to update kernel patch 1)
> > > >   
> > > 
> > >  >> what if pending_bytes is not 0, but vendor driver just does not want  to
> > >  >> send data in this iteration? isn't it right to get data_size first   
> > > before
> > >  >> getting data_offset?  
> > > 
> > > If vendor driver doesn't want to send data but still has data in staging 
> > > buffer, vendor driver still can control to send pending_bytes for this 
> > > iteration as 0 as this is a trap field.
> > > 
> > > I would defer this to Alex.
> > 
> > This is my understanding of the protocol as well, when the device is
> > running, pending_bytes might drop to zero if no internal state has
> > changed and may be non-zero on the next iteration due to device
> > activity.  When the device is not running, pending_bytes reporting zero
> > indicates the device is done, there is no further state to transmit.
> > Does that meet your need/expectation?
> >
> (1) on one side, as in vfio_save_pending(),
> vfio_save_pending()
> {
>     ...
>     ret = vfio_update_pending(vbasedev);
>     ...
>     *res_precopy_only += migration->pending_bytes;
>     ...
> }
> the pending_bytes tells migration thread how much data is still hold in
> device side.
> the device data includes
> device internal data + running device dirty data + device state.
> 
> so the pending_bytes should include device state as well, right?
> if so, the pending_bytes should never reach 0 if there's any device
> state to be sent after device is stopped.

I hadn't expected the pending-bytes to include a fixed offset for device
state (If you mean a few registers etc) - I'd expect pending to drop
possibly to zero;  the heuristic as to when to switch from iteration to
stop, is based on the total pending across all iterated devices; so it's
got to be allowed to drop otherwise you'll never transition to stop.

> (2) on the other side,
> along side we updated the pending_bytes in vfio_save_pending() and
> enter into the vfio_save_iterate(), if we repeatedly update
> pending_bytes in vfio_save_iterate(), it would enter into a scenario
> like
> 
> initially pending_bytes=500M.
> vfio_save_iterate() -->
>   round 1: transmitted 500M.
>   round 2: update pending bytes, pending_bytes=50M (50M dirty data).
>   round 3: update pending bytes, pending_bytes=50M.
>   ...
>   round N: update pending bytes, pending_bytes=50M.
> 
> If there're two vfio devices, the vfio_save_iterate() for the second device
> may never get chance to be called because there's always pending_bytes
> produced by the first device, even the size if small.

And between RAM and the vfio devices?

> > > > so I wonder which way in your mind is better, to give our reviewed-by to
> > > > the kernel part now, or hold until next qemu fixes?
> > > > and as performance data from gvt is requested from your previous mail, is
> > > > that still required before the code is accepted?
> > 
> > The QEMU series does not need to be perfect, I kind of expect we might
> > see a few iterations of that beyond the kernel portion being accepted.
> > We should have the QEMU series to the point that we've resolved any
> > uapi issues though, which it seems like we're pretty close to having.
> > Ideally I'd like to get the kernel series into my next branch before
> > the merge window opens, where it seems like upstream is on schedule to
> > have that happen this Sunday.  If you feel we're to the point were we
> > can iron a couple details out during the v5.8 development cycle, then
> > please provide your reviewed-by.  We haven't fully committed to a uapi
> > until we've committed to it for a non-rc release.
> > 
> got it.
> 
> > I think the performance request was largely due to some conversations
> > with Dave Gilbert wondering if all this actually works AND is practical
> > for a LIVE migration.  I think we're all curious about things like how
> > much data does a GPU have to transfer in each phase of migration, and
> > particularly if the final phase is going to be a barrier to claiming
> > the VM is actually sufficiently live.  I'm not sure we have many
> > options if a device simply has a very large working set, but even
> > anecdotal evidence that the stop-and-copy phase transfers abMB from the
> > device while idle or xyzMB while active would give us some idea what to
> for intel vGPU, the data is
> single-round dirty query:
> data to be transferred at stop-and-copy phase: 90MB+ ~ 900MB+, including
> - device state: 9MB
> - system dirty memory: 80MB+ ~ 900MB+ (depending on workload type)
> 
> multi-round dirty query :
> -each iteration data: 60MB ~ 400MB
> -data to be transferred at stop-and-copy phase: 70MB ~ 400MB
> 
> 
> 
> BTW, for viommu, the downtime data is as below. under the same network
> condition and guest memory size, and no running dirty data/memory produced
> by device.
> (1) viommu off
> single-round dirty query: downtime ~100ms 

Fine.

> (2) viommu on
> single-round dirty query: downtime 58s 

Youch.

Dave

> 
> Thanks
> Yan
> > expect.  Kirti, have you done any of those sorts of tests for NVIDIA's
> > driver?
> > 
> > > > BTW, we have also conducted some basic tests when viommu is on, and found out
> > > > errors like
> > > > "qemu-system-x86_64-dt: vtd_iova_to_slpte: detected slpte permission error (iova=0x0, level=0x3, slpte=0x0, write=1)
> > > > qemu-system-x86_64-dt: vtd_iommu_translate: detected translation failure (dev=00:03:00, iova=0x0)
> > > > qemu-system-x86_64-dt: New fault is not recorded due to compression of faults".
> > > >   
> > > 
> > > I saw these errors, I'm looking into it.
> > 
> > Let's try to at least determine if this is a uapi issue or just a QEMU
> > implementation bug for progressing the kernel series.  Thanks,
> > 
> > Alex
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK