On Thu, Apr 07, 2022 at 04:17:11PM +0100, Robin Murphy wrote: > For the specific case of overriding PCIe No Snoop (which is more problematic > from an Arm SMMU PoV) when assigning to a VM, would that not be easier > solved by just having vfio-pci clear the "Enable No Snoop" control bit in > the endpoint's PCIe capability? Ideally. That was rediscussed recently, apparently there are non-compliant devices and drivers that just ignore the bit. Presumably this is why x86 had to move to an IOMMU enforced feature.. > That seems a pretty good summary - I think they're basically all "firmware > told Linux I'm coherent so I'd better act coherent" cases, but that still > doesn't necessarily mean that they're *forced* to respect that. One of the > things on my to-do list is to try adding a DMA_ATTR_NO_SNOOP that can force > DMA cache maintenance for coherent devices, primarily to hook up in Panfrost > (where there is a bit of a performance to claw back on the coherent AmLogic > SoCs by leaving certain buffers non-cacheable). It would be great to see that in a way that could bring in the few other GPU drivers doing no-snoop to a formal DMA API instead of hacking their own stuff with wbinvd calls or whatever. Thanks, Jason
