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Message-ID: <58794e0e-43f5-f6cb-6b34-ad299ddac69f@intel.com>
Date:   Mon, 7 Feb 2022 11:13:04 -0800
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Subject: Re: [PATCH 0/3] selftests: Remove duplicate CPUID wrappers
Content-Language: en-US
To:     Shuah Khan <skhan@linuxfoundation.org>, <shuah@kernel.org>,
        <linux-kselftest@vger.kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ram Pai <linuxram@us.ibm.com>,
        Sandipan Das <sandipan@linux.ibm.com>,
        "Florian Weimer" <fweimer@redhat.com>,
        "Desnes A. Nunes do Rosario" <desnesn@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Thiago Jung Bauermann" <bauerman@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michal Suchanek <msuchanek@suse.de>, <linux-mm@kvack.org>,
        "Chang S . Bae" <chang.seok.bae@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>
References: <cover.1644000145.git.reinette.chatre@intel.com>
 <81df8c0e-fde6-f3b8-f988-b539f193635b@linuxfoundation.org>
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 <3787aee3-064a-dce7-1ab1-0dcabd56a100@linuxfoundation.org>
From:   Reinette Chatre <reinette.chatre@intel.com>
In-Reply-To: <3787aee3-064a-dce7-1ab1-0dcabd56a100@linuxfoundation.org>
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Hi Shuah,

On 2/7/2022 10:00 AM, Shuah Khan wrote:

> 
> This will work fine on newer versions of gcc/clang. However this could
> fail when mainline kselftest is used on stable releases on test rings
> and so on, especially if they have older versions of gcc/clang.

Indeed. It thus seems that kselftest has a minimal required version for
gcc/clang that is not the current mainline minimal version but the
minimal version of the oldest supported stable kernel, which is v4.9.

__cpuid_count() was added to gcc in commit:
cb0dee885cb30b4e9beeef070cf000baa7d09abe and thus available since
gcc 4.4.

Looking at Documentation/Changes or later Documentation/process/changes.rst
kernels v4.9 and v4.14 have the minimal required version of
gcc of 3.2. So this change would encounter an issue if mainline
kselftest is used to test a v4.9 or v4.14 kernel on a system that only
supports its minimal gcc.

Kernel v4.19 moved the gcc minimal required version to 4.6 that does
contain this macro.

There does not seem to be a minimum required version of clang/LLVM
in v4.19. The first time I see a minimal version for Clang/LLVM
for a stable kernel is in kernel v5.10 with Clang/LLVM minimal
version 10.0.1 and from what I can tell the __cpuid_count() macro
was added to Clang/LLVM in version 3.4.0 
(commit 4dcb5dbb53ea4fbeab48bc6bc3c4d392361dabc1).

> 
> We will have to find a solution for this. Instead of deleting the local
> define, let's keep it under ifndef __cpuid_count
> 
> /usr/lib/gcc/x86_64-linux-gnu/11/include/cpuid.h
> 
> #define __cpuid_count(level, count, a, b, c, d)                         \
>   __asm__ __volatile__ ("cpuid\n\t"                                     \
>                         : "=a" (a), "=b" (b), "=c" (c), "=d" (d)        \
>                         : "0" (level), "2" (count))
> 

Will do. I see that gcc obtained the volatile qualifier in v11.1 so I can use
the most recent macro as you have here.

Reinette