Messages in this thread | | | Date | Tue, 13 Oct 2020 10:44:16 +0800 | From | Shuo A Liu <> | Subject | Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces |
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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")
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