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

[Arvind Sankar]

On Mon, Oct 12, 2020 at 04:44:31PM +0800, Shuo A Liu wrote:
> On Wed 30.Sep'20 at 12:14:03 -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.
> >
> >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;
> >}
> 
> Yeah, this approach is also mentioned in the changelog. I will change to
> this way to follow your preference. With an addtional "r8" clobber what
> Arvind mentioned.
> 
> Thanks
> shuo

Btw, I noticed that arch/x86/xen/hypercall.h uses register-local
variables already for its hypercalls for quite some time, so this
wouldn't be unprecedented. [0]

Do these calls also need a memory clobber? The KVM/xen hypercall functions
all have one.

Thanks.

[0] e74359028d548 ("xen64: fix calls into hypercall page")