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

[Segher Boessenkool]

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