2017-08-17 16:48 GMT+08:00 Yang Zhang <yang.zhang.wz@xxxxxxxxx>: > On 2017/8/17 16:31, Wanpeng Li wrote: >> >> 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 > > > According to Paolo's comment "neither EPT violations nor misconfigurations > are listed in the manual among the VM exits that set the VM-exit instruction > length field", it seems to set the instruction length in vmcs12 is not right > though it is harmless. But Paolo also mentioned this "It just happens that the actual condition for VM-exit instruction length being set correctly is "the fault was taken after the accessing instruction has been decoded"." Regards, Wanpeng Li
