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

[Arvind Sankar]

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?

> 
> Segher