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
