Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO

[David Hildenbrand]

On 16.08.2017 15:34, 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 about a hack:

1. clear instruction length when entering
2. check if instruction length is set when trying to forward the RIP
2a. if set, use it
2b. if not set, compute it

this at least should give full speedup in existing setups. Not 99%
architecturally correct but might just work.

> 
> 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.
> 


-- 

Thanks,

David