KVM: x86: Default to not allowing emulation retry in kvm_mmu_page_fault

Effectively force kvm_mmu_page_fault() to opt-in to allowing retry to
make it more obvious when and why it allows emulation to be retried.
Previously this approach was less convenient due to retry and
re-execute behavior being controlled by separate flags that were also
inverted in their implementations (opt-in versus opt-out).

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0246a1ea7f55eb51e843e1ff586725d873da040b..01fb8701ccd02595552589a13c72052521c4f0ae 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5217,7 +5217,7 @@ static int make_mmu_pages_available(struct kvm_vcpu *vcpu)
 int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
                       void *insn, int insn_len)
 {
-       int r, emulation_type = EMULTYPE_ALLOW_RETRY;
+       int r, emulation_type = 0;
        enum emulation_result er;
        bool direct = vcpu->arch.mmu.direct_map;
 
@@ -5230,10 +5230,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
        r = RET_PF_INVALID;
        if (unlikely(error_code & PFERR_RSVD_MASK)) {
                r = handle_mmio_page_fault(vcpu, cr2, direct);
-               if (r == RET_PF_EMULATE) {
-                       emulation_type = 0;
+               if (r == RET_PF_EMULATE)
                        goto emulate;
-               }
        }
 
        if (r == RET_PF_INVALID) {
@@ -5260,8 +5258,16 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
                return 1;
        }
 
-       if (mmio_info_in_cache(vcpu, cr2, direct))
-               emulation_type = 0;
+       /*
+        * vcpu->arch.mmu.page_fault returned RET_PF_EMULATE, but we can still
+        * optimistically try to just unprotect the page and let the processor
+        * re-execute the instruction that caused the page fault.  Do not allow
+        * retrying MMIO emulation, as it's not only pointless but could also
+        * cause us to enter an infinite loop because the processor will keep
+        * faulting on the non-existent MMIO address.
+        */
+       if (!mmio_info_in_cache(vcpu, cr2, direct))
+               emulation_type = EMULTYPE_ALLOW_RETRY;
 emulate:
        /*
         * On AMD platforms, under certain conditions insn_len may be zero on #NPF.