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

[Kirti Wankhede <kwankhede@nvidia.com>]

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.

> 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?

> 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.

> 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?
> 
> 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.

Thanks,
Kirti