Re: [PATCH] x86: kvm: Avoid guest page table walk when gpa_available is set

[Brijesh Singh]

Hi Radim,


> >  	/* GPA available (AMD only) */
> >  	bool gpa_available;
> > +	gpa_t gpa_val;
>
> Can't we pass this information through function parameters?
>
> (I'd rather avoid intractable variables.)

I also wanted to avoid adding yet another variable but we can't depend on
cr2 parameters passed into x86_emulate_instruction().

The x86_emulate_instruction() function is called from two places:

1) handling the page-fault.
pf_interception [svm.c]
 kvm_mmu_page_fault [mmu.c]
  x86_emulate_instruction [x86.c]

2) completing the IO/MMIO's from previous instruction decode
kvm_arch_vcpu_ioctl_run
 complete_emulated_io
   emulate_instruction
    x86_emulate_instruction(vcpu, 0, emulation_type, NULL, 0)

In #1, we are guaranteed that cr2 variable will contain a valid GPA but
in #2, CR2 is set to zero.

> >  };
> >
> >  struct kvm_lpage_info {
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index 5fba706..8827e4b 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -4159,6 +4159,10 @@ static int handle_exit(struct kvm_vcpu *vcpu)
> >
> >  	vcpu->arch.gpa_available = (exit_code == SVM_EXIT_NPF);
> >
> > +	/* On #NPF, exit_info_2 contain a valid GPA */
> > +	if (vcpu->arch.gpa_available)
> > +		vcpu->arch.gpa_val = svm->vmcb->control.exit_info_2;
>
> How is vcpu->arch.gpa_val used between here and the NPF handler?

handle_exit [svm.c]
 pf_interception  [svm.c]
/* it invokes the fault handler with CR2 = svm->vmcb->control.exit_info_2 */
  kvm_mmu_page_fault [mmu.c]
   x86_emulate_instruction  [x86.c]
    emulator_read_write_onepage  [x86.c]
     /*
      *this is where we walk the guest page table to translate
      * a GVA to GPA. If gpa_available is set then we use the
      * gpa_val instead of walking the pgtable.
      */

> > +
> >  	if (!is_cr_intercept(svm, INTERCEPT_CR0_WRITE))
> >  		vcpu->arch.cr0 = svm->vmcb->save.cr0;
> >  	if (npt_enabled)
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > @@ -4653,18 +4653,16 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
> >  	 * occurred.
> >  	 */
> >  	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)) {
> > +		emulator_can_use_gpa(ctxt) &&
> > +		(addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) {
> >  		gpa = exception->address;
> > -		goto mmio;
> > +		ret = vcpu_is_mmio_gpa(vcpu, addr, gpa, write);
> > +	} else {
> > +		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;
> > @@ -5675,8 +5673,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> >  	}
> >
> >  restart:
> > -	/* Save the faulting GPA (cr2) in the address field */
> > -	ctxt->exception.address = cr2;
> > +	/*
> > +	 * Save the faulting GPA (cr2) in the address field
> > +	 * NOTE: If gpa_available is set then gpa_val will contain a valid GPA
> > +	 */
> > +	if (vcpu->arch.gpa_available)
> > +		ctxt->exception.address = vcpu->arch.gpa_val;
> > +	else
> > +		ctxt->exception.address = cr2;
>
> And related, shouldn't vcpu->arch.gpa_val be in cr2?

See my previous comment. In some cases CR2 may be set to zero
(e.g when completing the instruction from previous io/mmio page-fault).

If we are decide to add the gpa_val then we can remove above if
statement from x86_emulate_instruction() and update emulator_read_write_onepage
to use the vcpu->arch.gpa_val instead of exception->address.

if (vcpu->arch.gpa_available &&
     emulator_can_use_gpa(ctxt) &&
     (addr & ~PAGE_MASK) == (exception->address & ~PAGE_MASK)) {
      gpa = vcpu=>arch.gpa_val;
      ...
      ...
   }

-Brijesh