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

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Date:   Tue, 2 May 2017 18:24:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     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] x86: kvm: Avoid guest page table walk when gpa_available
 is set



On 28/04/2017 21:15, Brijesh Singh wrote:
> Hi Radim,
> 
>>
>> This will probably return false negatives when then vcpu->arch.gpa_val
>> couldn't be used anyway (possibly after a VM exits), so it is hard to
>> draw a conclusion.
>>
>> I would really like if we had a solution that passed the gpa inside
>> function parameters.
>>
>> (Btw. cr2 can also be a virtual address, so we can call it gpa directly)
>>
> 
> I've tried the below patch and it seems to work fine. This does not
> consider
> PIO case and as you rightly pointed PIO should trigger #NPF relatively
> rarely.
> At least so far in my runs I have not seen PIO causing #NPF. If this sounds
> acceptable approach then I can submit v2 with these changes and remove
> gpa_val
> additional.

PIO can certainly cause #NPF, albeit rarely, so this is not a viable one.

Paolo

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 13c132b..c040e38 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4662,17 +4662,17 @@ static int emulator_read_write_onepage(unsigned
> long addr, void *val,
>          */
>         if (vcpu->arch.gpa_available &&
>             emulator_can_use_gpa(ctxt) &&
> -           vcpu_is_mmio_gpa(vcpu, addr, exception->address, write) &&
>             (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) {
>                 gpa = exception->address;
> -               goto mmio;
> +               ret = vcpu_is_mmio_gpa(vcpu, addr, gpa, write);
> +       } else {
> +               dump_stack();
> +               ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception,
> write);
> +
> +               if (ret < 0)
> +                       return X86EMUL_PROPAGATE_FAULT;
>         }
>  
> -       ret = vcpu_mmio_gva_to_gpa(vcpu, addr, &gpa, exception, write);
> -
> -       if (ret < 0)
> -               return X86EMUL_PROPAGATE_FAULT;
> -
>         /* For APIC access vmexit */
>         if (ret)
>                 goto mmio;
> @@ -7056,11 +7056,11 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>         return r;
>  }
>  
> -static inline int complete_emulated_io(struct kvm_vcpu *vcpu)
> +static inline int complete_emulated_io(struct kvm_vcpu *vcpu, unsigned
> long cr2)
>  {
>         int r;
>         vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> -       r = emulate_instruction(vcpu, EMULTYPE_NO_DECODE);
> +       r = x86_emulate_instruction(vcpu, cr2, EMULTYPE_NO_DECODE, NULL,
> 0);
>         srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>         if (r != EMULATE_DONE)
>                 return 0;
> @@ -7071,7 +7071,7 @@ static int complete_emulated_pio(struct kvm_vcpu
> *vcpu)
>  {
>         BUG_ON(!vcpu->arch.pio.count);
>  
> -       return complete_emulated_io(vcpu);
> +       return complete_emulated_io(vcpu, 0);
>  }
>  /*
> @@ -7097,6 +7097,7 @@ static int complete_emulated_mmio(struct kvm_vcpu
> *vcpu)
>         struct kvm_run *run = vcpu->run;
>         struct kvm_mmio_fragment *frag;
>         unsigned len;
> +       gpa_t gpa;
>  
>         BUG_ON(!vcpu->mmio_needed);
>  
> @@ -7106,6 +7107,12 @@ static int complete_emulated_mmio(struct kvm_vcpu
> *vcpu)
>         if (!vcpu->mmio_is_write)
>                 memcpy(frag->data, run->mmio.data, len);
>  
> +       /*
> +       * lets use the GPA from previous guest page table walk to avoid yet
> +       * another guest page table walk when completing the MMIO
> page-fault.
> +        */
> +       gpa = frag->gpa;
> +
>         if (frag->len <= 8) {
>                 /* Switch to the next fragment. */
>                 frag++;
> @@ -7124,7 +7131,7 @@ static int complete_emulated_mmio(struct kvm_vcpu
> *vcpu)
>                 if (vcpu->mmio_is_write)
>                         return 1;
>                 vcpu->mmio_read_completed = 1;
> -               return complete_emulated_io(vcpu);
> +               return complete_emulated_io(vcpu, gpa);
>         }
>  
>         run->exit_reason = KVM_EXIT_MMIO;
>