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

[Arvind Sankar]

On Wed, Sep 30, 2020 at 03:59:15PM -0400, Arvind Sankar wrote:
> 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?

Also, I think you need memory clobbers for all of these in either case, no?