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

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Date:   Wed, 30 Sep 2020 19:11:22 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        shuo.a.liu@...el.com, LKML <linux-kernel@...r.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Yakui Zhao <yakui.zhao@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengwei Yin <fengwei.yin@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

On Wed, Sep 30, 2020 at 07:38:15PM -0400, Arvind Sankar wrote:
> 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?

Yes, x86 asm is hard to read, what can I say :-)  Sorry about that.


Segher