[PATCH v2 2/2] KVM: x86: On emulation failure, convey the exit reason to userspace

[David Edmondson]

Should instruction emulation fail, include the VM exit reason in the
emulation_failure data passed to userspace, in order that the VMM can
report it as a debugging aid when describing the failure.

Suggested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: David Edmondson <david.edmondson@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  2 ++
 arch/x86/kvm/vmx/vmx.c          |  5 +----
 arch/x86/kvm/x86.c              | 22 +++++++++++++---------
 include/uapi/linux/kvm.h        |  7 +++++++
 4 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0ee580c68839..2e411e26e40e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1630,6 +1630,8 @@ extern u64 kvm_mce_cap_supported;
 int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type);
 int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu,
 					void *insn, int insn_len);
+void kvm_prepare_emulation_failure_exit(struct kvm_vcpu *vcpu,
+					bool instruction_bytes);
 
 void kvm_enable_efer_bits(u64);
 bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d9a4d6cf6406..4fb240204c2c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5367,10 +5367,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
 
 		if (vmx->emulation_required && !vmx->rmode.vm86_active &&
 		    vcpu->arch.exception.pending) {
-			vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
-			vcpu->run->internal.suberror =
-						KVM_INTERNAL_ERROR_EMULATION;
-			vcpu->run->internal.ndata = 0;
+			kvm_prepare_emulation_failure_exit(vcpu, false);
 			return 0;
 		}
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 17468d983fbd..bf30b445b65d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7452,7 +7452,8 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
 }
 EXPORT_SYMBOL_GPL(kvm_inject_realmode_interrupt);
 
-static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
+void kvm_prepare_emulation_failure_exit(struct kvm_vcpu *vcpu,
+					bool instruction_bytes)
 {
 	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
 	u32 insn_size = ctxt->fetch.end - ctxt->fetch.data;
@@ -7463,7 +7464,7 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
 	run->emulation_failure.ndata = 0;
 	run->emulation_failure.flags = 0;
 
-	if (insn_size) {
+	if (insn_size && instruction_bytes) {
 		run->emulation_failure.ndata = 3;
 		run->emulation_failure.flags |=
 			KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES;
@@ -7473,7 +7474,14 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
 		memcpy(run->emulation_failure.insn_bytes,
 		       ctxt->fetch.data, insn_size);
 	}
+
+	run->emulation_failure.ndata = 4;
+	run->emulation_failure.flags |=
+		KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON;
+	run->emulation_failure.exit_reason =
+		static_call(kvm_x86_get_exit_reason)(vcpu);
 }
+EXPORT_SYMBOL_GPL(kvm_prepare_emulation_failure_exit);
 
 static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
 {
@@ -7489,16 +7497,14 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
 
 	if (kvm->arch.exit_on_emulation_error ||
 	    (emulation_type & EMULTYPE_SKIP)) {
-		prepare_emulation_failure_exit(vcpu);
+		kvm_prepare_emulation_failure_exit(vcpu, true);
 		return 0;
 	}
 
 	kvm_queue_exception(vcpu, UD_VECTOR);
 
 	if (!is_guest_mode(vcpu) && static_call(kvm_x86_get_cpl)(vcpu) == 0) {
-		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
-		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
-		vcpu->run->internal.ndata = 0;
+		kvm_prepare_emulation_failure_exit(vcpu, false);
 		return 0;
 	}
 
@@ -12092,9 +12098,7 @@ int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
 	 * doesn't seem to be a real use-case behind such requests, just return
 	 * KVM_EXIT_INTERNAL_ERROR for now.
 	 */
-	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
-	vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
-	vcpu->run->internal.ndata = 0;
+	kvm_prepare_emulation_failure_exit(vcpu, false);
 
 	return 0;
 }
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d9e4aabcb31a..863195371272 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -282,6 +282,7 @@ struct kvm_xen_exit {
 
 /* Flags that describe what fields in emulation_failure hold valid data. */
 #define KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES (1ULL << 0)
+#define KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON       (1ULL << 1)
 
 /* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
 struct kvm_run {
@@ -404,6 +405,12 @@ struct kvm_run {
 			__u64 flags;
 			__u8  insn_size;
 			__u8  insn_bytes[15];
+			/*
+			 * The "exit reason" extracted from the
+			 * VMCS/VMCB that was the cause of attempted
+			 * emulation.
+			 */
+			__u64 exit_reason;
 		} emulation_failure;
 		/* KVM_EXIT_OSI */
 		struct {
-- 
2.30.2