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

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

On Wed, 20 May 2020 19:10:07 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 5/20/2020 8:25 AM, 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 and Kirti,
> > after porting to qemu v22 and kernel v22, it is found out that
> > it can not even pass basic live migration test with error like
> > 
> > "Failed to get dirty bitmap for iova: 0xca000 size: 0x3000 err: 22"
> >   
> 
> Thanks for testing Yan.
> I think last moment change in below cause this failure
> 
> https://lore.kernel.org/kvm/1589871178-8282-1-git-send-email-kwankhede@nvidia.com/
> 
>  > 	if (dma->iova > iova + size)
>  > 		break;  
> 
> Surprisingly with my basic testing with 2G sys mem QEMU didn't raise 
> abort on g_free, but I do hit this with large sys mem.
> With above change, that function iterated through next vfio_dma as well. 
> Check should be as below:
> 
> -               if (dma->iova > iova + size)
> +               if (dma->iova > iova + size -1)


Or just:

	if (dma->iova >= iova + size)

Thanks,
Alex


>                          break;
> 
> Another fix is in QEMU.
> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg04751.html
> 
>  > > +        range->bitmap.size = ROUND_UP(pages, 64) / 8;  
>  >
>  > ROUND_UP(npages/8, sizeof(u64))?
>  >  
> 
> If npages < 8, npages/8 is 0 and ROUND_UP(0, 8) returns 0.
> 
> Changing it as below
> 
> -        range->bitmap.size = ROUND_UP(pages / 8, sizeof(uint64_t));
> +        range->bitmap.size = ROUND_UP(pages, sizeof(__u64) * 
> BITS_PER_BYTE) /
> +                             BITS_PER_BYTE;
> 
> I'm updating patches with these fixes and Cornelia's suggestion soon.
> 
> Due to short of time I may not be able to address all the concerns 
> raised on previous versions of QEMU, I'm trying make QEMU side code 
> available for testing for others with latest kernel changes. Don't 
> worry, I will revisit comments on QEMU patches. Right now first priority 
> is to test kernel UAPI and prepare kernel patches for 5.8
> 
> Thanks,
> Kirti
>