Messages in this thread | | | Date | Mon, 12 Oct 2020 16:44:31 +0800 | From | Shuo A Liu <> | Subject | Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces |
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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
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