2017-08-17 16:07 GMT+08:00 Yang Zhang <yang.zhang.wz@xxxxxxxxx>: > On 2017/8/17 0:56, Radim Krčmář wrote: >> >> 2017-08-16 17:10+0300, Michael S. Tsirkin: >>> >>> On Wed, Aug 16, 2017 at 03:34:54PM +0200, Paolo Bonzini wrote: >>>> >>>> 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 way >>>> to keep the full speedup. >>>> >>>> What we can do is use EMULTYPE_SKIP; it only saves 200 clock cycles >>>> because computing the physical RIP and reading the instruction is >>>> expensive, but at least the eventfd is signaled before entering the >>>> emulator. This saves on latency. While at it, don't check breakpoints >>>> when skipping the instruction, as presumably any side effect has been >>>> exposed already. >>>> >>>> Adding a hypercall or MSR write that does a fast MMIO write to a >>>> physical >>>> address would do it, but it adds hypervisor knowledge in virtio, >>>> including >>>> CPUID handling. So it would be pretty ugly in the guest-side >>>> implementation, >>>> but if somebody wants to do it and the virtio side is acceptable to the >>>> virtio maintainers, I am okay with it. >>>> >>>> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> >>>> Cc: stable@xxxxxxxxxxxxxxx >>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae >>>> Suggested-by: Radim Krčmář <rkrcmar@xxxxxxxxxx> >>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> >>> >>> >>> Jason (cc) who worked on the original optimization said he can >>> work to test the performance impact. >>> I suggest we don't rush this (it's been like this for 2 years), >>> and the issue seems to be largely theoretical. >> >> >> Paolo, did Microsoft point it out because they hit the bug when running >> KVM on Hyper-V? > > > Does this mean the nested emulation of EPT violation and misconfiguration in > KVM side doesn't strictly follow the manual since we didn't hit the bug in > KVM? The VM-exit instruction length of vmcs12 is provided by vmcs02 (prepare_vmcs12()), so unless the length from vmcs02 is wrong. In addition, something like mov instruction which can trigger the EPT violation/misconfig in guest has already been decoded before executing I think, IIUC, then exit qualification can have the information about the instruction length. Regards, Wanpeng Li
