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

[Arvind Sankar]

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).

How strict is the guarantee? This is an inline function -- could the
compiler decide to reorder some other code in between the r8 assignment
and the asm statement when it gets inlined?

> >
> > 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;
> }

This seems more robust, though you probably need an r8 clobber in there?
Is hcall_id actually just 32 bits or can it be >=2^32?