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

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Message-ID: <20200930171346.GC2628@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Sep 2020 19:13:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        shuo.a.liu@...el.com, linux-kernel@...r.kernel.org, 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>, ndesaulniers@...gle.com
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

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..