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

Arvind Sankar
2020-09-30T23:38:21Z

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