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

Brijesh Singh
2017-04-24T22:14:21Z

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