Use input constraints to load RAX and RBX when testing that KVM correctly
does/doesn't patch the "wrong" hypercall.  There's no need to manually
load RAX and RBX, and no reason to clobber them either (KVM is not
supposed to modify anything other than RAX).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 .../selftests/kvm/x86_64/fix_hypercall_test.c | 22 +++++++------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
index 2512df357ab3..dde97be3e719 100644
--- a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
+++ b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
@@ -30,14 +30,11 @@ static uint64_t svm_do_sched_yield(uint8_t apic_id)
 {
 	uint64_t ret;
 
-	asm volatile("mov %1, %%rax\n\t"
-		     "mov %2, %%rbx\n\t"
-		     "svm_hypercall_insn:\n\t"
+	asm volatile("svm_hypercall_insn:\n\t"
 		     "vmmcall\n\t"
-		     "mov %%rax, %0\n\t"
-		     : "=r"(ret)
-		     : "r"((uint64_t)KVM_HC_SCHED_YIELD), "r"((uint64_t)apic_id)
-		     : "rax", "rbx", "memory");
+		     : "=a"(ret)
+		     : "a"((uint64_t)KVM_HC_SCHED_YIELD), "b"((uint64_t)apic_id)
+		     : "memory");
 
 	return ret;
 }
@@ -47,14 +44,11 @@ static uint64_t vmx_do_sched_yield(uint8_t apic_id)
 {
 	uint64_t ret;
 
-	asm volatile("mov %1, %%rax\n\t"
-		     "mov %2, %%rbx\n\t"
-		     "vmx_hypercall_insn:\n\t"
+	asm volatile("vmx_hypercall_insn:\n\t"
 		     "vmcall\n\t"
-		     "mov %%rax, %0\n\t"
-		     : "=r"(ret)
-		     : "r"((uint64_t)KVM_HC_SCHED_YIELD), "r"((uint64_t)apic_id)
-		     : "rax", "rbx", "memory");
+		     : "=a"(ret)
+		     : "a"((uint64_t)KVM_HC_SCHED_YIELD), "b"((uint64_t)apic_id)
+		     : "memory");
 
 	return ret;
 }
-- 
2.37.2.789.g6183377224-goog

On Thu, Sep 08, 2022 at 11:31:32PM +0000, Sean Christopherson wrote:
> Use input constraints to load RAX and RBX when testing that KVM correctly
> does/doesn't patch the "wrong" hypercall.  There's no need to manually
> load RAX and RBX, and no reason to clobber them either (KVM is not
> supposed to modify anything other than RAX).

Too much time on 'the other architecture' where we don't have input
constraints to load named registers per-se :)

> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

--
Thanks,
Oliver