KVM: selftests: Provide error code as a KVM_ASM_SAFE() output

Provide the error code on a fault in KVM_ASM_SAFE(), e.g. to allow tests
to assert that #PF generates the correct error code without needing to
manually install a #PF handler.  Use r10 as the scratch register for the
error code, as it's already clobbered by the asm blob (loaded with the
RIP of the to-be-executed instruction).  Deliberately load the output
"error_code" even in the non-faulting path so that error_code is always
initialized with deterministic data (the aforementioned RIP), i.e to
ensure a selftest won't end up with uninitialized consumption regardless
of how KVM_ASM_SAFE() is used.

Don't clear r10 in the non-faulting case and instead load error code with
the RIP (see above).  The error code is valid if and only if an exception
occurs, and '0' isn't necessarily a better "invalid" value, e.g. '0'
could result in false passes for a buggy test.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20221102184654.282799-9-dmatlack@google.com

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -786,6 +786,7 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
  *
  * REGISTER OUTPUTS:
  * r9  = exception vector (non-zero)
+ * r10 = error code
  */
 #define KVM_ASM_SAFE(insn)					\
 	"mov $" __stringify(KVM_EXCEPTION_MAGIC) ", %%r9\n\t"	\
@@ -794,17 +795,30 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
 	"1: " insn "\n\t"					\
 	"xor %%r9, %%r9\n\t"					\
 	"2:\n\t"						\
-	"mov  %%r9b, %[vector]\n\t"
+	"mov  %%r9b, %[vector]\n\t"				\
+	"mov  %%r10, %[error_code]\n\t"
 
-#define KVM_ASM_SAFE_OUTPUTS(v)	[vector] "=qm"(v)
+#define KVM_ASM_SAFE_OUTPUTS(v, ec)	[vector] "=qm"(v), [error_code] "=rm"(ec)
 #define KVM_ASM_SAFE_CLOBBERS	"r9", "r10", "r11"
 
 #define kvm_asm_safe(insn, inputs...)					\
 ({									\
+	uint64_t ign_error_code;					\
 	uint8_t vector;							\
 									\
 	asm volatile(KVM_ASM_SAFE(insn)					\
-		     : KVM_ASM_SAFE_OUTPUTS(vector)	\
+		     : KVM_ASM_SAFE_OUTPUTS(vector, ign_error_code)	\
+		     : inputs						\
+		     : KVM_ASM_SAFE_CLOBBERS);				\
+	vector;								\
+})
+
+#define kvm_asm_safe_ec(insn, error_code, inputs...)			\
+({									\
+	uint8_t vector;							\
+									\
+	asm volatile(KVM_ASM_SAFE(insn)					\
+		     : KVM_ASM_SAFE_OUTPUTS(vector, error_code)		\
 		     : inputs						\
 		     : KVM_ASM_SAFE_CLOBBERS);				\
 	vector;								\
@@ -812,11 +826,12 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
 
 static inline uint8_t rdmsr_safe(uint32_t msr, uint64_t *val)
 {
+	uint64_t error_code;
 	uint8_t vector;
 	uint32_t a, d;
 
 	asm volatile(KVM_ASM_SAFE("rdmsr")
-		     : "=a"(a), "=d"(d), KVM_ASM_SAFE_OUTPUTS(vector)
+		     : "=a"(a), "=d"(d), KVM_ASM_SAFE_OUTPUTS(vector, error_code)
 		     : "c"(msr)
 		     : KVM_ASM_SAFE_CLOBBERS);
 

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -1068,6 +1068,7 @@ static bool kvm_fixup_exception(struct ex_regs *regs)
 
 	regs->rip = regs->r11;
 	regs->r9 = regs->vector;
+	regs->r10 = regs->error_code;
 	return true;
 }
 

tools/testing/selftests/kvm/x86_64/hyperv_features.c

@@ -18,6 +18,7 @@
 static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address,
 				vm_vaddr_t output_address, uint64_t *hv_status)
 {
+	uint64_t error_code;
 	uint8_t vector;
 
 	/* Note both the hypercall and the "asm safe" clobber r9-r11. */
@@ -25,7 +26,7 @@ static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address,
 		     KVM_ASM_SAFE("vmcall")
 		     : "=a" (*hv_status),
 		       "+c" (control), "+d" (input_address),
-		       KVM_ASM_SAFE_OUTPUTS(vector)
+		       KVM_ASM_SAFE_OUTPUTS(vector, error_code)
 		     : [output_address] "r"(output_address),
 		       "a" (-EFAULT)
 		     : "cc", "memory", "r8", KVM_ASM_SAFE_CLOBBERS);