* Alex Williamson (alex.william...@redhat.com) wrote: > On Tue, 27 Apr 2021 17:29:37 +0100 > Dev Audsin <dev.devaq...@gmail.com> wrote: > > > Hi Alex > > > > Based on your comments and thinking a bit, wonder if it makes sense to > > allow DMA map for the DAX cache but make unexpected mappings to be not > > fatal. Please let me know your thoughts. > > I think you're still working on the assumption that simply making the > VM boot is an improvement, it's not. If there's a risk that a possible > DMA target for the device cannot be mapped, it's better that the VM > fail to boot than to expose that risk. Performance cannot compromise > correctness. > > We do allow DMA mappings to other device memory regions to fail > non-fatally with the logic that peer-to-peer DMA is often not trusted > to work by drivers and therefore support would be probed before > assuming that it works. I don't think that same logic applies here. > > Is there something about the definition of this particular region that > precludes it from being a DMA target for an assigned devices?
It's never really the ram that's used. This area is really a chunk of VMA that's mmap'd over by (chunks of) normal files in the underlying exported filesystem. The actual RAM block itself is just a placeholder for the VMA, and is normally mapped PROT_NONE until an actual file is mapped on top of it. That cache bar is a mapping containing multiple separate file chunk mappings. So I guess the problems for VFIO are: a) At the start it's unmapped, unaccessible, unallocated ram. b) Later it's arbitrary chunks of ondisk files. [on a bad day, and it's bad even without vfio, someone truncates the file mapping] Dave > Otherwise if it's initially unpopulated, maybe something like the > RamDiscardManager could be used to insert DMA mappings as the region > becomes populated. > > Simply disabling mapping to boot with both features together, without > analyzing how that missing mapping affects their interaction is not > acceptable. Thanks, > > Alex > > > On Mon, Apr 26, 2021 at 10:22 PM Alex Williamson > > <alex.william...@redhat.com> wrote: > > > > > > On Mon, 26 Apr 2021 21:50:38 +0100 > > > Dev Audsin <dev.devaq...@gmail.com> wrote: > > > > > > > Hi Alex and David > > > > > > > > @Alex: > > > > > > > > Justification on why this region cannot be a DMA target for the device, > > > > > > > > virtio-fs with DAX is currently not compatible with NIC Pass through. > > > > When a SR-IOV VF attaches to a qemu process, vfio will try to pin the > > > > entire DAX Window but it is empty when the guest boots and will fail. > > > > A method to make VFIO and DAX to work together is to make vfio skip > > > > DAX cache. > > > > > > > > Currently DAX cache need to be set to 0, for the SR-IOV VF to be > > > > attached to Kata containers. Enabling both SR-IOV VF and DAX work > > > > together will potentially improve performance for workloads which are > > > > I/O and network intensive. > > > > > > Sorry, there's no actual justification described here. You're enabling > > > a VM with both features, virtio-fs DAX and VFIO, but there's no > > > evidence that they "work together" or that your use case is simply > > > avoiding a scenario where the device might attempt to DMA into the area > > > with this designation. With this change, if the device were to attempt > > > to DMA into this region, it would be blocked by the IOMMU, which might > > > result in a data loss within the VM. Justification of this change > > > needs to prove that this region can never be a DMA target for the > > > device, not simply that both features can be enabled and we hope that > > > they don't interact. Thanks, > > > > > > Alex > > > > > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
