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

Shuo A Liu
2020-10-12T08:44:42Z

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@infradead.org> 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