Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

Segher Boessenkool
2020-10-01T00:17:42Z

On Wed, Sep 30, 2020 at 07:38:15PM -0400, Arvind Sankar wrote:
> On Wed, Sep 30, 2020 at 06:25:59PM -0500, Segher Boessenkool wrote:
> > On Wed, Sep 30, 2020 at 12:14:03PM -0700, Nick Desaulniers wrote:
> > > Do we need register local storage here?
> >
> > Depends what you want. It looks like you do:
> >
> > > static inline long bar(unsigned long hcall_id)
> > > {
> > > long result;
> > > asm volatile("movl %1, %%r8d\n\t"
> > > "vmcall\n\t"
> > > : "=a" (result)
> > > : "ir" (hcall_id)
> > > : );
> > > return result;
> > > }
> >
> > "result" as output from the asm is in %rax, and the compiler will
> > shuffle that to wherever it needs it as the function return value. That
> > part will work fine.
> >
> > But how you are accessing %r8d is not correct, that needs to be a local
> > register asm (or r8 be made a fixed reg, probably not what you want ;-) )
>
> Doesn't it just need an "r8" clobber to allow using r8d?

Yes, x86 asm is hard to read, what can I say :-) Sorry about that.

Segher