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Sender: Arvind Sankar <niveditas98@gmail.com>
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Mon, 12 Oct 2020 12:49:16 -0400
To:     Shuo A Liu <shuo.a.liu@intel.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Yu Wang <yu1.wang@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Yakui Zhao <yakui.zhao@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Fengwei Yin <fengwei.yin@intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>,
        Zhenyu Wang <zhenyuw@linux.intel.com>
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces
Message-ID: <20201012164916.GA613777@rani.riverdale.lan>
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 <20200930171346.GC2628@hirez.programming.kicks-ass.net>
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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.

Thanks.

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