On Wed, 2 Jun 2021 13:01:40 -0300 Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > On Wed, Jun 02, 2021 at 02:20:15AM +0000, Tian, Kevin wrote: > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > Sent: Wednesday, June 2, 2021 6:22 AM > > > > > > On Tue, 1 Jun 2021 07:01:57 +0000 > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > > > > > > > I summarized five opens here, about: > > > > > > > > 1) Finalizing the name to replace /dev/ioasid; > > > > 2) Whether one device is allowed to bind to multiple IOASID fd's; > > > > 3) Carry device information in invalidation/fault reporting uAPI; > > > > 4) What should/could be specified when allocating an IOASID; > > > > 5) The protocol between vfio group and kvm; > > > > > > > ... > > > > > > > > For 5), I'd expect Alex to chime in. Per my understanding looks the > > > > original purpose of this protocol is not about I/O address space. It's > > > > for KVM to know whether any device is assigned to this VM and then > > > > do something special (e.g. posted interrupt, EPT cache attribute, etc.). > > > > > > Right, the original use case was for KVM to determine whether it needs > > > to emulate invlpg, so it needs to be aware when an assigned device is > > > > invlpg -> wbinvd :) Oops, of course. > > > present and be able to test if DMA for that device is cache > > > coherent. > > Why is this such a strong linkage to VFIO and not just a 'hey kvm > emulate wbinvd' flag from qemu? IIRC, wbinvd has host implications, a malicious user could tell KVM to emulate wbinvd then run the op in a loop and induce a disproportionate load on the system. We therefore wanted a way that it would only be enabled when required. > I briefly didn't see any obvios linkage in the arch code, just some > dead code: > > $ git grep iommu_noncoherent > arch/x86/include/asm/kvm_host.h: bool iommu_noncoherent; > $ git grep iommu_domain arch/x86 > arch/x86/include/asm/kvm_host.h: struct iommu_domain *iommu_domain; > > Huh? Cruft from legacy KVM device assignment, I assume. What you're looking for is: kvm_vfio_update_coherency kvm_arch_register_noncoherent_dma atomic_inc(&kvm->arch.noncoherent_dma_count); need_emulate_wbinvd kvm_arch_has_noncoherent_dma atomic_read(&kvm->arch.noncoherent_dma_count); There are a couple other callers that I'm not as familiar with. > It kind of looks like the other main point is to generate the > VFIO_GROUP_NOTIFY_SET_KVM which is being used by two VFIO drivers to > connect back to the kvm data > > But that seems like it would have been better handled with some IOCTL > on the vfio_device fd to import the KVM to the driver not this > roundabout way? Then QEMU would need to know which drivers require KVM knowledge? This allowed transparent backwards compatibility with userspace. Thanks, Alex
