Re: [PATCH 4/5] selftests/kvm/x86_64: set rax before vmcall

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Andrei Vagin <avagin@google.com> writes:

> kvm_hypercall has to place the hypercall number in rax.
>
> Trace events show that kvm_pv_test doesn't work properly:
>      kvm_pv_test-53132: kvm_hypercall: nr 0x0 a0 0x0 a1 0x0 a2 0x0 a3 0x0
>      kvm_pv_test-53132: kvm_hypercall: nr 0x0 a0 0x0 a1 0x0 a2 0x0 a3 0x0
>      kvm_pv_test-53132: kvm_hypercall: nr 0x0 a0 0x0 a1 0x0 a2 0x0 a3 0x0
>
> With this change, it starts working as expected:
>      kvm_pv_test-54285: kvm_hypercall: nr 0x5 a0 0x0 a1 0x0 a2 0x0 a3 0x0
>      kvm_pv_test-54285: kvm_hypercall: nr 0xa a0 0x0 a1 0x0 a2 0x0 a3 0x0
>      kvm_pv_test-54285: kvm_hypercall: nr 0xb a0 0x0 a1 0x0 a2 0x0 a3 0x0
>

Fixes: ac4a4d6de22e ("selftests: kvm: test enforcement of paravirtual cpuid features")

> Signed-off-by: Andrei Vagin <avagin@google.com>
> ---
>  tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index ead7011ee8f6..5d85e1c021da 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -1422,7 +1422,7 @@ uint64_t kvm_hypercall(uint64_t nr, uint64_t a0, uint64_t a1, uint64_t a2,
>  
>  	asm volatile("vmcall"
>  		     : "=a"(r)
> -		     : "b"(a0), "c"(a1), "d"(a2), "S"(a3));
> +		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3));

Wouldn't '"+a"(r)' instead of '"=a"(r)' suffice (assuming we also assing
'r' to 'nr' in the beginning, something like

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index ead7011ee8f6..fdd6554b94a1 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -1418,10 +1418,10 @@ bool set_cpuid(struct kvm_cpuid2 *cpuid,
 uint64_t kvm_hypercall(uint64_t nr, uint64_t a0, uint64_t a1, uint64_t a2,
                       uint64_t a3)
 {
-       uint64_t r;
+       uint64_t r = nr;
 
        asm volatile("vmcall"
-                    : "=a"(r)
+                    : "+a"(r)
                     : "b"(a0), "c"(a1), "d"(a2), "S"(a3));
        return r;
 }

-- 
Vitaly