From: Sean Christopherson <[email protected]>
Explicitly check for an MMIO spte in the fast page fault flow. TDX will
use a not-present entry for MMIO sptes, which can be mistaken for an
access-tracked spte since both have SPTE_SPECIAL_MASK set.
MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
patch does not affect them. TDX will handle MMIO emulation through a
hypercall instead.
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Isaku Yamahata <[email protected]>
---
arch/x86/kvm/mmu/mmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 17252f39bd7c..51306b80f47c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3163,7 +3163,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
else
sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
- if (!is_shadow_present_pte(spte))
+ if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
break;
sp = sptep_to_sp(sptep);
--
2.25.1
On Mon, 2022-06-27 at 14:53 -0700, [email protected] wrote:
> From: Sean Christopherson <[email protected]>
>
> Explicitly check for an MMIO spte in the fast page fault flow. TDX will
> use a not-present entry for MMIO sptes, which can be mistaken for an
> access-tracked spte since both have SPTE_SPECIAL_MASK set.
SPTE_SPECIAL_MASK has been removed in latest KVM code. The changelog needs
update.
In fact, if I understand correctly, I don't think this changelog is correct:
The existing code doesn't check is_mmio_spte() because:
1) If MMIO caching is enabled, MMIO fault is always handled in
handle_mmio_page_fault() before reaching here;
2) If MMIO caching is disabled, is_shadow_present_pte() always returns false for
MMIO spte, and is_mmio_spte() also always return false for MMIO spte, so there's
no need check here.
"A non-present entry for MMIO spte" doesn't necessarily mean
is_shadow_present_pte() will return true for it, and there's no explanation at
all that for TDX guest a MMIO spte could reach here and is_shadow_present_pte()
returns true for it.
If this patch is ever needed, it should come with or after the patch (patches)
that handles MMIO fault for TD guest.
Hi Sean, Paolo,
Did I miss anything?
>
> MMIO sptes are handled in handle_mmio_page_fault for non-TDX VMs, so this
> patch does not affect them. TDX will handle MMIO emulation through a
> hypercall instead.
>
> Signed-off-by: Sean Christopherson <[email protected]>
> Signed-off-by: Isaku Yamahata <[email protected]>
> ---
> arch/x86/kvm/mmu/mmu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 17252f39bd7c..51306b80f47c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3163,7 +3163,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> else
> sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
>
> - if (!is_shadow_present_pte(spte))
> + if (!is_shadow_present_pte(spte) || is_mmio_spte(spte))
> break;
>
> sp = sptep_to_sp(sptep);
On Thu, Jun 30, 2022 at 11:37:15PM +1200,
Kai Huang <[email protected]> wrote:
> On Mon, 2022-06-27 at 14:53 -0700, [email protected] wrote:
> > From: Sean Christopherson <[email protected]>
> >
> > Explicitly check for an MMIO spte in the fast page fault flow. TDX will
> > use a not-present entry for MMIO sptes, which can be mistaken for an
> > access-tracked spte since both have SPTE_SPECIAL_MASK set.
>
> SPTE_SPECIAL_MASK has been removed in latest KVM code. The changelog needs
> update.
It was renamed to SPTE_TDP_AD_MASK. not removed.
> In fact, if I understand correctly, I don't think this changelog is correct:
> The existing code doesn't check is_mmio_spte() because:
>
> 1) If MMIO caching is enabled, MMIO fault is always handled in
> handle_mmio_page_fault() before reaching here;
>
> 2) If MMIO caching is disabled, is_shadow_present_pte() always returns false for
> MMIO spte, and is_mmio_spte() also always return false for MMIO spte, so there's
> no need check here.
>
> "A non-present entry for MMIO spte" doesn't necessarily mean
> is_shadow_present_pte() will return true for it, and there's no explanation at
> all that for TDX guest a MMIO spte could reach here and is_shadow_present_pte()
> returns true for it.
Although it was needed, I noticed the following commit made this patch
unnecessary. So I'll drop this patch. Kudos to Sean.
edea7c4fc215c7ee1cc98363b016ad505cbac9f7
"KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs"
--
Isaku Yamahata <[email protected]>
>
> Although it was needed, I noticed the following commit made this patch
> unnecessary. So I'll drop this patch. Kudos to Sean.
>
> edea7c4fc215c7ee1cc98363b016ad505cbac9f7
> "KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs"
>
Yes is_shadow_present_pte() always return false for MMIO so this patch isn't
needed anymore.