On Mon, Dec 18, 2023 at 07:13:27AM -0800, Sean Christopherson wrote: >On Mon, Dec 18, 2023, Tao Su wrote: >> When host doesn't support 5-level EPT, bits 51:48 of the guest physical >> address must all be zero, otherwise an EPT violation always occurs and >> current handler can't resolve this if the gpa is in RAM region. Hence, >> instruction will keep being executed repeatedly, which causes infinite >> EPT violation. >> >> Six KVM selftests are timeout due to this issue: >> kvm:access_tracking_perf_test >> kvm:demand_paging_test >> kvm:dirty_log_test >> kvm:dirty_log_perf_test >> kvm:kvm_page_table_test >> kvm:memslot_modification_stress_test >> >> The above selftests add a RAM region close to max_gfn, if host has 52 >> physical bits but doesn't support 5-level EPT, these will trigger infinite >> EPT violation when access the RAM region. >> >> Since current Intel CPUID doesn't report max guest physical bits like AMD, >> introduce kvm_mmu_tdp_maxphyaddr() to limit guest physical bits when tdp is >> enabled and report the max guest physical bits which is smaller than host. >> >> When guest physical bits is smaller than host, some GPA are illegal from >> guest's perspective, but are still legal from hardware's perspective, >> which should be trapped to inject #PF. Current KVM already has a parameter >> allow_smaller_maxphyaddr to support the case when guest.MAXPHYADDR < >> host.MAXPHYADDR, which is disabled by default when EPT is enabled, user >> can enable it when loading kvm-intel module. When allow_smaller_maxphyaddr >> is enabled and guest accesses an illegal address from guest's perspective, >> KVM will utilize EPT violation and emulate the instruction to inject #PF >> and determine #PF error code. > >No, fix the selftests, it's not KVM's responsibility to advertise the correct >guest.MAXPHYADDR. In this case, host.MAXPHYADDR is 52 and EPT supports 4-level only thus can translate up to 48 bits of GPA. Here nothing visible to selftests or QEMU indicates that guest.MAXPHYADDR = 52 is invalid/incorrect. how can we say selftests are at fault and we should fix them?
