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Sender: Arvind Sankar <niveditas98@gmail.com>
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Wed, 30 Sep 2020 19:38:15 -0400
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
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        "H . Peter Anvin" <hpa@zytor.com>,
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        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Fengwei Yin <fengwei.yin@intel.com>,
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        Zhenyu Wang <zhenyuw@linux.intel.com>
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces
Message-ID: <20200930233815.GA3372783@rani.riverdale.lan>
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On Wed, Sep 30, 2020 at 06:25:59PM -0500, Segher Boessenkool wrote:
> On Wed, Sep 30, 2020 at 12:14:03PM -0700, Nick Desaulniers wrote:
> > Do we need register local storage here?
> 
> Depends what you want.  It looks like you do:
> 
> > 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;
> > }
> 
> "result" as output from the asm is in %rax, and the compiler will
> shuffle that to wherever it needs it as the function return value.  That
> part will work fine.
> 
> But how you are accessing %r8d is not correct, that needs to be a local
> register asm (or r8 be made a fixed reg, probably not what you want ;-) )
> 

Doesn't it just need an "r8" clobber to allow using r8d?

> 
> Segher