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

Bill Wendling
2019-09-11T22:35:24Z

On Wed, Sep 11, 2019 at 2:08 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> 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 :-).
>
No prob. :-) I'm not familiar with sending these patches via email. Do
I need to regenerate the patch and send via "send-email"? (Similar
question for the "Reviewed-By" comments.)

> Out of curiosity, do any compilers actually generate errors because of
> this, or is it simply dead code?
>
Yeah, clang uses a memory location, probably due to inlining causing
register pressure (that's a guess, I didn't explore it too deeply).

-bw

> > 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;
> > }