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

Brijesh Singh
2017-04-28T19:15:48Z

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.

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;