[PATCH v12 35/84] KVM: x86: Use kvm_lookup_pfn() to check if retrying #PF is useful

Sean Christopherson
2024-07-27T00:07:17Z

Use kvm_lookup_pfn() instead of an open coded equivalent when checking to
see if KVM should exit to userspace or re-enter the guest after failed
instruction emulation triggered by a guest page fault.

Note, there is a small functional change as kvm_lookup_pfn() doesn't mark
the page as accessed, whereas kvm_release_pfn_clean() does mark the page
accessed (if the pfn is backed by a refcounted struct page). Neither
behavior is wrong per se, e.g. querying the gfn=>pfn mapping doesn't
actually access the page, but the guest _did_ access the gfn, otherwise
the fault wouldn't have occurred.

That said, either KVM will exit to userspace and the guest will likely be
terminated, or KVM will re-enter the guest and, barring weirdness in the
guest, the guest will re-access the gfn, and KVM will fault-in the pfn and
mark it accessed.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/x86.c | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index af6c8cf6a37a..59501ad6e7f5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8867,7 +8867,6 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
int emulation_type)
{
gpa_t gpa = cr2_or_gpa;
- kvm_pfn_t pfn;

if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
return false;
@@ -8892,22 +8891,15 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
}

/*
- * Do not retry the unhandleable instruction if it faults on the
- * readonly host memory, otherwise it will goto a infinite loop:
+ * Do not retry the unhandleable instruction if emulation was triggered
+ * for emulated MMIO, e.g. by a readonly memslot or lack of a memslot,
+ * otherwise KVM will send the vCPU into an infinite loop:
* retry instruction -> write #PF -> emulation fail -> retry
* instruction -> ...
*/
- pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
-
- /*
- * If the instruction failed on the error pfn, it can not be fixed,
- * report the error to userspace.
- */
- if (is_error_noslot_pfn(pfn))
+ if (is_error_noslot_pfn(kvm_lookup_pfn(vcpu->kvm, gpa_to_gfn(gpa))))
return false;

- kvm_release_pfn_clean(pfn);
-
/*
* If emulation may have been triggered by a write to a shadowed page
* table, unprotect the gfn (zap any relevant SPTEs) and re-enter the