On Fri, Oct 23, 2020 at 2:22 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 23/10/20 05:14, Sean Christopherson wrote: > >>>> + > >>>> + /* > >>>> + * Check that the GPA doesn't exceed physical memory limits, as that is > >>>> + * a guest page fault. We have to emulate the instruction here, because > >>>> + * if the illegal address is that of a paging structure, then > >>>> + * EPT_VIOLATION_ACC_WRITE bit is set. Alternatively, if supported we > >>>> + * would also use advanced VM-exit information for EPT violations to > >>>> + * reconstruct the page fault error code. > >>>> + */ > >>>> + if (unlikely(kvm_mmu_is_illegal_gpa(vcpu, gpa))) > >>>> + return kvm_emulate_instruction(vcpu, 0); > >>>> + > >>> Is kvm's in-kernel emulator up to the task? What if the instruction in > >>> question is AVX-512, or one of the myriad instructions that the > >>> in-kernel emulator can't handle? Ice Lake must support the advanced > >>> VM-exit information for EPT violations, so that would seem like a > >>> better choice. > >>> > >> Anyone? > > > > Using "advanced info" if it's supported seems like the way to go. Outright > > requiring it is probably overkill; if userspace wants to risk having to kill a > > (likely broken) guest, so be it. > > Yeah, the instruction is expected to page-fault here. However the > comment is incorrect and advanced information does not help here. > > The problem is that page fault error code bits cannot be reconstructed > from bits 0..2 of the EPT violation exit qualification, if bit 8 is > clear in the exit qualification (that is, if the access causing the EPT > violation is to a paging-structure entry). In that case bits 0..2 refer > to the paging-structure access rather than to the final access. In fact > advanced information is not available at all for paging-structure access > EPT violations. True, but the in-kernel emulator can only handle a very small subset of the available instructions. If bit 8 is set in the exit qualification, we should use the advanced VM-exit information. If it's clear, we should just do a software page walk of the guest's x86 page tables. The in-kernel emulator should only be used as a last resort on hardware that doesn't support the advanced VM-exit information for EPT violations.
