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

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Date:   Thu, 17 Aug 2017 16:48:55 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, "# v3 . 10+" <stable@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast
 MMIO

On 2017/8/17 16:31, Wanpeng Li wrote:
> 2017-08-17 16:28 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
>> 2017-08-17 16:07 GMT+08:00 Yang Zhang <yang.zhang.wz@...il.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@...hat.com>
>>>>>> Cc: stable@...r.kernel.org
>>>>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
>>>>>> Suggested-by: Radim Krčmář <rkrcmar@...hat.com>
>>>>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.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?
>>
>> 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.
>
> s/exit qualification/VM-exit instruction length

According to Paolo's comment "neither EPT violations nor 
misconfigurations are listed in the manual among the VM exits that set 
the VM-exit instruction length field", it seems to set the instruction 
length in vmcs12 is not right though it is harmless.

-- 
Yang
Alibaba Cloud Computing