linux-kernel - Re: [PATCH] x86/acrn: Improve ACRN hypercalls

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Date:   Thu, 4 Aug 2022 11:41:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/acrn: Improve ACRN hypercalls

On 8/4/22 11:03, Uros Bizjak wrote:
> As explained in section 6.47.5.2, "Specifying Registers for Local Variables"
> of the GCC info documentation, the correct way to specify register for
> input operands when calling Extended 'asm' is to define a local register
> variable and associate it with a specified register:
> 
> 	register unsigned long r8 asm ("r8") = hcall_id;

IIRC, that's what the ACRN folks proposed first.  But, it's more fragile
because you can't, for instance, put a printk() in that function between
the variable definition and assebly.