On 16.08.2017 14:07, Radim Krčmář wrote: > 2017-08-16 13:22+0200, Paolo Bonzini: >> Microsoft pointed out privately to me that KVM's handling of >> KVM_FAST_MMIO_BUS is invalid. Using skip_emulation_instruction is invalid >> in EPT misconfiguration vmexit handlers, because neither EPT violations >> nor misconfigurations are listed in the manual among the VM exits that >> set the VM-exit instruction length field. >> >> While physical processors seem to set the field, this is not architectural >> and is just a side effect of the implementation. I couldn't convince >> myself of any condition on the exit qualification where VM-exit >> instruction length "has" to be defined; there are no trap-like VM-exits >> that can be repurposed; and fault-like VM-exits such as descriptor-table >> exits provide no decoding information. So I don't really see any elegant >> way to fix it except by disabling KVM_FAST_MMIO_BUS, which means virtio >> 1 will go slower. > > Do you have some numbers? > > We could keep the ugliness in KVM and add a new skip function with > emulate_instruction(vcpu, EMULTYPE_SKIP) to decode the length of the > instruction. (Adding a condition just for EPT violation exit reason to I like that idea. > the existing skip function would be a dirtier solution.) > Slower than what we have now, but faster than full emulation. > > I agree that configuring EPT to throw a violation when accessing fast > MMIO has many drawbacks (although it seems to be what Intel expected). > > Thanks. -- Thanks, David