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

[Yang Zhang <yang.zhang.wz@gmail.com>]

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