linux-kernel - Re: [PATCH v2] KVM: x86: Avoid guest page table walk when gpa

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Date:   Tue, 8 Aug 2017 14:24:59 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     brijesh.singh@....com, pbonzini@...hat.com, joro@...tes.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, Thomas.Lendacky@....com
Subject: Re: [PATCH v2] KVM: x86: Avoid guest page table walk when
 gpa_available is set

Hi Radim,


On 07/20/2017 02:43 AM, Radim Krčmář wrote:
> 2017-07-19 08:35-0500, Brijesh Singh:
>> On 07/19/2017 06:19 AM, Radim Krčmář wrote:
>>> 2017-07-17 16:32-0500, Brijesh Singh:
>>>> Hi Paolo and Radim
>>>>
>>>> Any comments on this patch, I could not find it in 4.13-2 branch.
>>>>
>>>> Please let me know if you want to fix something, or want me to
>>>> refresh and resend the patch.
>>>
>>> Sorry, I tried it during the merge window, but it didn't pass tests on
>>> VMX and I got distracted by other bugs before looking into the cause.
>>>
>>> Can you reproduce the fail?
>>>
>>
>> No worries, thanks.
>>
>> I can try to reproduce it, are you running kvm-unittest or something different?
> 
> I noticed that a linux guest hung in early boot, but at least (io)apic
> kvm-unit-tests failed as well, IIRC.
> 
>> IIRC, VMX does not set the gpa_available flag hence I am wondering what did I miss
>> in the patch to trigger the failure. I will debug it and let you know.
> 
> It does now, in ept_violation and ept_misconfig,
> 

I am able to reproduce the issue on VMX, Sorry it took a bit longer to verify
it.

I was not aware that VMX is also making use of gpa_available flag hence I missed
updating the vmx.c to set the gpa_val. After applying the below small patch I am
able to boot the guest on Intel Xeon E5-2665.

Additionally, there was one issue in current patch pointed by Paolo [1]. If patch
was using vcpu->arch.gpa_val check as pointed by Paolo then on VMX we will silently
fallback to guest page table walk (even when gpa_available is set). I guess since I
have testing my code on SVM platform hence never caught the error. I will soon send
updated patch.

[1] http://marc.info/?l=kvm&m=150116338725964&w=2

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index b5e0b02..9309fbb 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6309,6 +6309,7 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
                       ? PFERR_PRESENT_MASK : 0;
  
         vcpu->arch.gpa_available = true;
+       vcpu->arch.gpa_val = gpa;
         vcpu->arch.exit_qualification = exit_qualification;
  
         return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
@@ -6326,6 +6327,7 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
         }
  
         ret = handle_mmio_page_fault(vcpu, gpa, true);
+       vcpu->arch.gpa_val = gpa;
         vcpu->arch.gpa_available = true;
         if (likely(ret == RET_MMIO_PF_EMULATE))
                 return x86_emulate_instruction(vcpu, gpa, 0, NULL, 0) ==