Re: [kvm-unit-tests PATCH] x86: remove memory constraint from "mov" instruction

Sean Christopherson
2019-09-11T19:08:42Z

On Mon, Sep 09, 2019 at 02:19:20PM -0700, Bill Wendling wrote:
> The "mov" instruction to get the error code shouldn't move into a memory
> location. Don't allow the compiler to make this decision. Instead
> specify that only a register is appropriate here.

I'd prefer the changelog to say something like:

Remove a bogus memory contraint as x86 does not have a generic
memory-to-memory "mov" instruction.

Saying "shouldn't move into a memory location" makes it sound like there's
an unwanted side effect when the compiler selects memory, though I suppose
you could argue that a build error is an unwanted side effect :-).

Out of curiosity, do any compilers actually generate errors because of
this, or is it simply dead code?

>
> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
> lib/x86/desc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/x86/desc.c b/lib/x86/desc.c
> index 5f37cef..451f504 100644
> --- a/lib/x86/desc.c
> +++ b/lib/x86/desc.c
> @@ -263,7 +263,7 @@ unsigned exception_error_code(void)
> {
> unsigned short error_code;
>
> - asm("mov %%gs:6, %0" : "=rm"(error_code));
> + asm("mov %%gs:6, %0" : "=r"(error_code));
> return error_code;
> }