linux-kernel - Re: [PATCH] KVM: nVMX: Fix exception injection

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Date:   Sun, 23 Jul 2017 08:29:58 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: nVMX: Fix exception injection

2017-07-22 22:25 GMT+08:00 Jim Mattson <jmattson@...gle.com>:
> On Fri, Jul 21, 2017 at 1:39 AM, Wanpeng Li <kernellwp@...il.com> wrote:
>> Hi Jim,
>> 2017-07-21 3:16 GMT+08:00 Jim Mattson <jmattson@...gle.com>:
>>> On Wed, Jul 19, 2017 at 7:31 PM, Wanpeng Li <kernellwp@...il.com> wrote:
>>>> Hi Jim,
>>>> 2017-07-19 2:47 GMT+08:00 Jim Mattson <jmattson@...gle.com>:
>>>>> Why do we expect the VM_EXIT_INTR_INFO and EXIT_QUALIFICATION fields
>>>>> of the VMCS to have the correct values for the injected exception?
>>>>
>>>> Good point, I think we should synthesize VM_EXIT_INTR_INFO and
>>>> EXIT_QUALIFICATION manually, I will post a patch for it. Btw, how
>>>> about setting EXIT_QULIFICATION to vcpu->arch.cr2 for the page fault
>>>> exception and 0 for other exceptions?
>>>
>>> From the SDM, section 27.1:
>>>
>>>     If an event causes a VM exit directly, it does not update
>>
>> I mentioned this in the patch description:
>>
>>> However, there is no guarantee the exit reason is exception currently, when there is an external interrupt occurred on host, maybe a time interrupt for host which should not be injected to guest, and somewhere queues an exception, then the function nested_vmx_check_exception() will be called and the vmexit emulation codes will try to emulate the "Acknowledge interrupt on exit" behavior, the warning is triggered.
>>
>> If you think the scenario is correct, then it should be an event
>> causes a VM exit indirectly. So if both the scenario which I mentioned
>> and "This function
>> assumes it is called with the exit reason in vmcs02 being a #PF
>> exception" can happen, then maybe we should figure out how to fix both
>> scenarios suitable.
>
> In the situation you describe, the #PF causes a synthesized VM-exit
> from L2 to L1 directly, not indirectly. From the SDM:
>
>    An exception causes a VM exit directly if the bit corresponding to
> that exception is set in the exception bitmap.
>
> Hence, CR2 should not be set yet.

Any idea how to synthesize exit qualification for page fault and debug
exception?

Regards,
Wanpeng Li

>
>>
>>> architectural state as it would have if it had it not caused the VM
>>> exit:
>>>       - A debug exception does not update DR6, DR7.GD, or
>>> IA32_DEBUGCTL.LBR. (Information about the nature of the debug
>>> exception is saved in the exit qualification field.)
>>>       - A page fault does not update CR2. (The linear address causing
>>> the page fault is saved in the exit-qualification field.)
>>>
>>> This means that vcpu->arch.cr2 should not be set at this point for a
>>> #PF injection (and vcpu->arch.dr6 should not be set at this point for
>>> a #DB injection). For all other exceptions, yes, the exit
>>> qualification should be cleared.
>>>