Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum

From: Andy Lutomirski <luto@amacapital.net>
To: Borislav Petkov <bp@alien8.de>
Cc: Junichi Nomura <j-nomura@ce.jp.nec.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"hpa@zytor.com" <hpa@zytor.com>
Subject: Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum
Date: Fri, 30 Dec 2016 18:13:24 -0800	[thread overview]
Message-ID: <CALCETrUcaEyj3sDtEHSseJs5ZuKbX6sz_bskfmDH+WSKWf92qg@mail.gmail.com> (raw)
In-Reply-To: <20161229093012.GA11221@nazgul.tnic>

On Thu, Dec 29, 2016 at 1:30 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Wed, Dec 28, 2016 at 10:11:22AM -0800, Andy Lutomirski wrote:
>> On a very quick read, it looks like none of your new call sites
>> actually use the return value at all.  Since you also appear to be
>> consolidating them all, would it make sense to just open-code the
>> single (?) remaining user?
>
> I've got stuff coming up which will use the retval but it is not fully
> cooked yet. And also, we want to have generic helpers so that people do
> not reimplement them left and right.

Okay, but I still think that a variant that says "do cpuid and ignore
the return value" would make sense.  Imagine a very clever
implementation of native_cpuid_eax like:

asm ("cpuid" : "=a" (eax) : ...);
return eax;

Now you call it and ignore the return value and the compiler optimizes
it out :)  Also, someone reading the code might scratch their head and
wonder why you picked eax and not ebx, ecx, or edx.

--Andy