[PATCH 0/4] Update cpu feature extraction

From: vladimir.murzin@arm•com (Vladimir Murzin)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 0/4] Update cpu feature extraction
Date: Tue, 19 Apr 2016 12:39:10 +0100	[thread overview]
Message-ID: <571618DE.5050201@arm.com> (raw)
In-Reply-To: <56AB40AB.8070302@arm.com>

On 29/01/16 10:36, Vladimir Murzin wrote:
> On 28/01/16 17:01, Russell King - ARM Linux wrote:
>> On Thu, Jan 28, 2016 at 11:13:10AM +0000, Vladimir Murzin wrote:
>>> On 26/01/16 09:23, Vladimir Murzin wrote:
>>>> Hi,
>>>>
>>>> This has been started as a fix in cpu feature extractor, but Suzuki suggested
>>>> syncing-up the whole cpu feature handling with the recent updates on arm64
>>>> side [1,2].
>>>>
>>>> I've kept fixes separately, so they can go as the are in case there is concern
>>>> on updating cpu feature handling.
>>>>
>>>> [1] http://comments.gmane.org/gmane.linux.ports.arm.kernel/455568
>>>> [2] https://lkml.org/lkml/2015/11/18/570
>>>>
>>>
>>> Russell, is it OK for patch tracker?
>>
>> I'd really like to hear that someone has validated these changes or at
>> least someone in ARM Ltd such as Will or Catalin has reviewed them; I
>> remember the subject of whether certain fields are signed or unsigned
>> being almost a personal opinion - we used to treat all fields as
>> unsigned, but that was declared to be wrong, so we then went to treating
>> them all as signed fields... See:
>>
>> commit b8c9592b4a6c93211c8163888a97880d608503b5
>> Author: Ard Biesheuvel <ard.biesheuvel@linaro•org>
>> Date:   Thu Mar 19 19:03:25 2015 +0100
>>
>>     ARM: 8318/1: treat CPU feature register fields as signed quantities
>>
>>     The various CPU feature registers consist of 4-bit blocks that
>>     represent signed quantities, whose positive values represent
>>     incremental features, and whose negative values are reserved.
>>
>>     To improve forward compatibility, update the feature detection
>>     code to take possible future higher values into account, but
>>     ignore negative values.
>>
>>     Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro•org>
>>     Signed-off-by: Russell King <rmk+kernel@arm•linux.org.uk>
>>
>> For instance:
>>
>> -       sync_prim = ((read_cpuid_ext(CPUID_EXT_ISAR3) >> 8) & 0xf0) |
>> -                   ((read_cpuid_ext(CPUID_EXT_ISAR4) >> 20) & 0x0f);
>> -       if (sync_prim >= 0x13)
>> +       if (cpuid_feature_extract(CPUID_EXT_ISAR3, 12) > 1 ||
>> +           (cpuid_feature_extract(CPUID_EXT_ISAR3, 12) == 1 &&
>> +            cpuid_feature_extract(CPUID_EXT_ISAR3, 20) >= 3))
>>                 elf_hwcap &= ~HWCAP_SWP;
>>
>> (let's ignore the ISAR4 vs ISAR3 problem).  What if ISAR4 20:23 have
>> bit 23 set?  In the original code, that would be treated as being ">= 3"
>> but in the new code, that's less than zero, so would fail the test.
>>
>> I think we had much the same questions back in March 2015 when this was
>> last discussed, when I raised the issue of there being a documentation
>> bug in that the documentation doesn't make it clear how reserved 4-bit
>> CPU ID fields should be intepreted.
>>
>> What I'd like to see is some kind of positive concensus on this (and
>> a doc update); what I don't want to do is to apply these patches and
>> then get another round of patches converting some of them back to
>> signed tests, and then later another set of patches doing the reverse,
>> because that seems to be what's now happening.
>>
> 
> It is fair point. Since this topic is quite fuzzy and discussion can
> take some time I'd be happy if only fixes (the first two patches) can
> find their way to mainline independent of the rest of the series.
> 
> Is it OK for patches 1/4 and 2/4 to go in patch tracker?
> 

I've just uploaded patches 1/4 and 2/4 to patch tracker with assumption
they are OK, please, let me know in case it's wrong.

Cheers
Vladimir

> Thanks
> Vladimir
> 
> 
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