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 16:28:01 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Yang Zhang <yang.zhang.wz@...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

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.

Regards,
Wanpeng Li