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

[David Edmondson <david.edmondson@xxxxxxxxxx>]

To aid in debugging.

Suggested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: David Edmondson <david.edmondson@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c       | 23 +++++++++++++++++------
 include/uapi/linux/kvm.h |  2 ++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8166ad113fb2..48ef0dc68faf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7455,7 +7455,7 @@ 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)
+static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu, uint64_t flags)
 {
 	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
 	u32 insn_size = ctxt->fetch.end - ctxt->fetch.data;
@@ -7466,7 +7466,8 @@ 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 &&
+	    (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES)) {
 		run->emulation_failure.ndata = 3;
 		run->emulation_failure.flags |=
 			KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES;
@@ -7476,6 +7477,14 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
 		memcpy(run->emulation_failure.insn_bytes,
 		       ctxt->fetch.data, insn_size);
 	}
+
+	if (flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON) {
+		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);
+	}
 }
 
 static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
@@ -7492,16 +7501,18 @@ 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);
+		prepare_emulation_failure_exit(
+			vcpu,
+			KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES |
+			KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);
 		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;
+		prepare_emulation_failure_exit(
+			vcpu, KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON);
 		return 0;
 	}
 
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 68c9e6d8bbda..3e4126652a67 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 +405,7 @@ struct kvm_run {
 			__u64 flags;
 			__u8  insn_size;
 			__u8  insn_bytes[15];
+			__u64 exit_reason;
 		} emulation_failure;
 		/* KVM_EXIT_OSI */
 		struct {
-- 
2.30.2