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SubjectRe: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO
2017-08-18 16:46+0800, Jason Wang:
>
>
> 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%

Might be noticeable ... I'm ok with the hypervisor detection workaround.

Still, we will need a replacement mechanism for virtio if Intel doesn't
change SDM. And drop this workaround after a solution has been
implemented.

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