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

Shuo A Liu
2020-10-13T02:44:25Z

On Mon 12.Oct'20 at 12:49:16 -0400, Arvind Sankar wrote:
>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@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
>
>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.

Yes. it's needed. I will add it. Thanks

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