Messages in this thread |  | | Subject | Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO | From | Yang Zhang <> | Date | Thu, 17 Aug 2017 16:07:17 +0800 |
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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@redhat.com> >>> Cc: stable@vger.kernel.org >>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae >>> Suggested-by: Radim Krčmář <rkrcmar@redhat.com> >>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> >> >> 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?
> > Thanks. >
-- Yang Alibaba Cloud Computing
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