[lkml]   [2017]   [Aug]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO
2017-08-17 16:07 GMT+08:00 Yang Zhang <>:
> 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 <>
>>>> Cc:
>>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
>>>> Suggested-by: Radim Krčmář <>
>>>> Signed-off-by: Paolo Bonzini <>
>>> 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?

The VM-exit instruction length of vmcs12 is provided by vmcs02
(prepare_vmcs12()), so unless the length from vmcs02 is wrong. In
addition, something like mov instruction which can trigger the EPT
violation/misconfig in guest has already been decoded before executing
I think, IIUC, then exit qualification can have the information about
the instruction length.

Wanpeng Li

 \ /
  Last update: 2017-08-17 10:28    [W:0.054 / U:1.132 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site