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

[Michael S. Tsirkin]

On Thu, Dec 08, 2022 at 09:58:06AM -0500, Peter Xu wrote:
> On Thu, Dec 08, 2022 at 08:48:09AM +0100, Eric Auger wrote:
> > Hi Peter,
> 
> Hi, Eric,
> 
> > 
> > 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.
> 
> It's not the race in the code, it's the race between modifying host IOMMU
> pgtable with DMA happening in parallel.  The bug triggered with DMA_MAP
> returning -EEXIST means there's existing mapping.
> 
> If during replay there's mapped ranges and the ranges are prone to DMA,
> then IIUC it can happen.
> 
> I didn't really check specifically for virtio-iommu and I mostly forget the
> details, just to raise this up.  It's possible for some reason it just
> can't trigger.  VT-d definitely can, in which case we'll see DMA errors on
> the host from the assigned device when the DMA triggers during the "unmap
> and map" window.
> 
> Thanks,

Eric any resolution on this?

> -- 
> Peter Xu