Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 30 Sep 2020 19:13:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        shuo.a.liu@intel.com, linux-kernel@vger.kernel.org, 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>, ndesaulniers@google.com
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces
Message-ID: <20200930171346.GC2628@hirez.programming.kicks-ass.net>
References: <20200922114311.38804-1-shuo.a.liu@intel.com>
 <20200922114311.38804-5-shuo.a.liu@intel.com>
 <20200927105152.GG88650@kroah.com>
 <6f9a2b83-6904-2290-6c4f-526672390beb@intel.com>
 <20200930111612.GZ2628@hirez.programming.kicks-ass.net>
 <20200930161036.GY28786@gate.crashing.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200930161036.GY28786@gate.crashing.org>
Precedence: bulk

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..