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Date:   Tue, 13 Oct 2020 10:44:16 +0800
From:   Shuo A Liu <shuo.a.liu@intel.com>
To:     Arvind Sankar <nivedita@alum.mit.edu>
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: <20201013024416.GN1057@shuo-intel.sh.intel.com>
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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")