2017-08-17 16:28 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>: > 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. s/exit qualification/VM-exit instruction length
