On Tue, 17 Dec 2019 02:28:33 +0000 "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > From: Paolo Bonzini > > Sent: Monday, December 16, 2019 6:08 PM > > > > [Alex and Kevin: there are doubts below regarding dirty page tracking > > from VFIO and mdev devices, which perhaps you can help with] > > > > On 15/12/19 18:21, Peter Xu wrote: > > > init_rmode_tss > > > vmx_set_tss_addr > > > kvm_vm_ioctl_set_tss_addr [*] > > > init_rmode_identity_map > > > vmx_create_vcpu [*] > > > > These don't matter because their content is not visible to userspace > > (the backing storage is mmap-ed by __x86_set_memory_region). In fact, d > > > > > vmx_write_pml_buffer > > > kvm_arch_write_log_dirty [&] > > > kvm_write_guest > > > kvm_hv_setup_tsc_page > > > kvm_guest_time_update [&] > > > nested_flush_cached_shadow_vmcs12 [&] > > > kvm_write_wall_clock [&] > > > kvm_pv_clock_pairing [&] > > > kvmgt_rw_gpa [?] > > > > This then expands (partially) to > > > > intel_gvt_hypervisor_write_gpa > > emulate_csb_update > > emulate_execlist_ctx_schedule_out > > complete_execlist_workload > > complete_current_workload > > workload_thread > > emulate_execlist_ctx_schedule_in > > prepare_execlist_workload > > prepare_workload > > dispatch_workload > > workload_thread > > > > So KVMGT is always writing to GPAs instead of IOVAs and basically > > bypassing a guest IOMMU. So here it would be better if kvmgt was > > changed not use kvm_write_guest (also because I'd probably have nacked > > that if I had known :)). > > I agree. > > > > > As far as I know, there is some work on live migration with both VFIO > > and mdev, and that probably includes some dirty page tracking API. > > kvmgt could switch to that API, or there could be VFIO APIs similar to > > kvm_write_guest but taking IOVAs instead of GPAs. Advantage: this would > > fix the GPA/IOVA confusion. Disadvantage: userspace would lose the > > tracking of writes from mdev devices. Kevin, are these writes used in > > any way? Do the calls to intel_gvt_hypervisor_write_gpa covers all > > writes from kvmgt vGPUs, or can the hardware write to memory as well > > (which would be my guess if I didn't know anything about kvmgt, which I > > pretty much don't)? > > intel_gvt_hypervisor_write_gpa covers all writes due to software mediation. > > for hardware updates, it needs be mapped in IOMMU through vfio_pin_pages > before any DMA happens. The ongoing dirty tracking effort in VFIO will take > every pinned page through that API as dirtied. > > However, currently VFIO doesn't implement any vfio_read/write_guest > interface yet. and it doesn't make sense to use vfio_pin_pages for software > dirtied pages, as pin is unnecessary and heavy involving iommu invalidation. > > Alex, if you are OK we'll work on such interface and move kvmgt to use it. > After it's accepted, we can also mark pages dirty through this new interface > in Kirti's dirty page tracking series. I'm not sure what you're asking for, is it an interface for the host CPU to read/write the memory backing of a mapped IOVA range without pinning pages? That seems like something like that would make sense for an emulation model where a page does not need to be pinned for physical DMA. If you're asking more for an interface that understands the userspace driver is a VM (ie. implied using a _guest postfix on the function name) and knows about GPA mappings beyond the windows directly mapped for device access, I'd not look fondly on such a request. Thanks, Alex
