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

[Peter Zijlstra]

On Wed, Sep 30, 2020 at 12:14:03PM -0700, Nick Desaulniers wrote:
> 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.

That only tells me it compiles it, not if that (IMO) weird construct is
actually guaranteed to work as expected.

I'd almost dive into the GCC archives to read the back-story to this
'feature', it just seems to weird to me. A well, for another day that.