Re: [PATCH for 8.0 0/2] virtio-iommu: Fix Replay

[Eric Auger]

Hi Peter,

On 12/8/22 00:49, Peter Xu wrote:
> Hi, Eric,
>
> On Wed, Dec 07, 2022 at 02:36:44PM +0100, Eric Auger wrote:
>> When assigning VFIO devices protected by a virtio-iommu we need to replay
>> the mappings when adding a new IOMMU MR and when attaching a device to
>> a domain. While we do a "remap" we currently fail to first unmap the
>> existing IOVA mapping and just map the new one. With some device/group
>> topology this can lead to errors in VFIO when trying to DMA_MAP IOVA
>> ranges onto existing ones.
> I'm not sure whether virtio-iommu+vfio will suffer from DMA races like when
> we were working on the vt-d replay for vfio.  The issue is whether DMA can
> happen right after UNMAP but before MAP of the same page if the page was
> always mapped.

I don't think it can race because a mutex is hold while doing the
virtio_iommu_replay(), and each time a virtio cmd is handled (attach,
map, unmap), see virtio_iommu_handle_command.
So I think it is safe.

Thanks

Eric
>
> The vt-d resolved it by using iova_tree so in a replay vt-d knows the page
> didn't change, so it avoids unmap+map.  It only notifies newly unmapped or
> newly mapped.
>
> Thanks,
>