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

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum
Date	2016-12-31 03:20 +0100
Message-ID	<sUhr3-5cP-1@gated-at.bofh.it> (permalink)
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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

Thread

[PATCH] x86: Fix Intel microcode revision detection Junichi Nomura <j-nomura@ce.jp.nec.com> - 2016-12-28 05:50 +0100
  [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single  datum Borislav Petkov <bp@alien8.de> - 2016-12-28 12:30 +0100
    Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum Andy Lutomirski <luto@amacapital.net> - 2016-12-28 19:20 +0100
      Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a  single datum Borislav Petkov <bp@alien8.de> - 2016-12-29 10:40 +0100
        Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a single datum Andy Lutomirski <luto@amacapital.net> - 2016-12-31 03:20 +0100
          Re: [PATCH 1/2] x86/CPU: Add native CPUID variants returning a  single datum Borislav Petkov <bp@alien8.de> - 2016-12-31 12:20 +0100
  [PATCH 2/2] x86/microcode: Use native CPUID to tickle out microcode  revision Borislav Petkov <bp@alien8.de> - 2016-12-28 12:30 +0100
    [PATCH 3/2] x86/microcode/intel: Add a helper which gives the  microcode revision Borislav Petkov <bp@alien8.de> - 2016-12-28 14:00 +0100
      Re: [PATCH 3/2] x86/microcode/intel: Add a helper which gives the  microcode revision Andy Lutomirski <luto@amacapital.net> - 2016-12-28 19:20 +0100
        Re: [PATCH 3/2] x86/microcode/intel: Add a helper which gives the  microcode revision Borislav Petkov <bp@alien8.de> - 2016-12-29 10:40 +0100
      Re: [PATCH 3/2] x86/microcode/intel: Add a helper which gives the  microcode revision Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-12-28 20:30 +0100
        Re: [PATCH 3/2] x86/microcode/intel: Add a helper which gives the  microcode revision Borislav Petkov <bp@alien8.de> - 2016-12-29 10:40 +0100
  Re: [PATCH] x86: Fix Intel microcode revision detection Borislav Petkov <bp@alien8.de> - 2016-12-28 12:30 +0100

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