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

[Nick Desaulniers]

On Wed, Sep 30, 2020 at 10:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Sep 30, 2020 at 11:10:36AM -0500, Segher Boessenkool wrote:
>
> > Since this variable is a local register asm, on entry to the asm the
> > compiler guarantees that the value lives in the assigned register (the
> > "r8" hardware register in this case).  This all works completely fine.
> > This is the only guaranteed behaviour for local register asm (well,
> > together with analogous behaviour for outputs).
>
> Right, that's what they're trying to achieve. The hypervisor calling
> convention needs that variable in %r8 (which is somewhat unfortunate).
>
> AFAIK this is the first such use in the kernel, but at least the gcc-4.9
> (our oldest supported version) claims to support this.
>
> So now we need to know if clang will actually do this too..

Does clang support register local storage? Let's use godbolt.org to find out:
https://godbolt.org/z/YM45W5
Looks like yes. You can even check different GCC versions via the
dropdown in the top right.

The -ffixed-* flags are less well supported in Clang; they need to be
reimplemented on a per-backend basis. aarch64 is relatively well
supported, but other arches not so much IME.

Do we need register local storage here?

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;
}
-- 
Thanks,
~Nick Desaulniers