KVM: selftests: Remove unnecessary register shuffling in fix_hypercall_test

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>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Message-Id: <20220928233652.783504-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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 e0004bd..6864eb0 100644 (file)
--- 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;
 }