Messages in this thread |  | | Subject | Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO | From | Jason Wang <> | Date | Fri, 18 Aug 2017 16:46:38 +0800 |
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On 2017年08月16日 22:10, Michael S. Tsirkin wrote: > 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 see regressions on both latency and cpu utilization through netperf TCP_RR test:
pkt_size/sessions/+transaction_rate%/+per_cpu_transaction_rate% 1/ 1/ +0%/ -5% 1/ 25/ -1%/ -2% 1/ 50/ -9%/ -10% 64/ 1/ -3%/ -9% 64/ 25/ 0%/ -2% 64/ 50/ -10%/ -11% 256/ 1/ -10%/ -17% 256/ 25/ -11%/ -12% 256/ 50/ -9%/ -11%
Thanks
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