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

[Eric Auger]

Hi Michael, Peter,
On 12/20/22 15:59, Michael S. Tsirkin wrote:
> 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?

Sorry for the delay. Not yet unfortunately. following Peter's reply I
now understand the race issue and it makes total sense but I need to
study it further.

Eric


>
>> -- 
>> Peter Xu