linux-kernel - Re: [PATCH] crypto: x86/crc32c-intel

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Date:   Thu, 7 Jan 2021 14:23:15 +0800
From:   Tony W Wang-oc <TonyWWang-oc@...oxin.com>
To:     <hpa@...or.com>, Eric Biggers <ebiggers@...nel.org>
CC:     <herbert@...dor.apana.org.au>, <davem@...emloft.net>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <x86@...nel.org>, <linux-crypto@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <TimGuo-oc@...oxin.com>,
        <CooperYan@...oxin.com>, <QiyuanWang@...oxin.com>,
        <HerryYang@...oxin.com>, <CobeChen@...oxin.com>,
        <SilviaZhao@...oxin.com>
Subject: Re: [PATCH] crypto: x86/crc32c-intel - Don't match some Zhaoxin CPUs


On 22/12/2020 12:54, hpa@...or.com wrote:
> On December 21, 2020 7:01:39 PM PST, tonywwang-oc@...oxin.com wrote:
>> On December 22, 2020 3:27:33 AM GMT+08:00, hpa@...or.com wrote:
>>> On December 20, 2020 6:46:25 PM PST, tonywwang-oc@...oxin.com wrote:
>>>> On December 16, 2020 1:56:45 AM GMT+08:00, Eric Biggers
>>>> <ebiggers@...nel.org> wrote:
>>>>> On Tue, Dec 15, 2020 at 10:15:29AM +0800, Tony W Wang-oc wrote:
>>>>>>
>>>>>> On 15/12/2020 04:41, Eric Biggers wrote:
>>>>>>> On Mon, Dec 14, 2020 at 10:28:19AM +0800, Tony W Wang-oc wrote:
>>>>>>>> On 12/12/2020 01:43, Eric Biggers wrote:
>>>>>>>>> On Fri, Dec 11, 2020 at 07:29:04PM +0800, Tony W Wang-oc
>> wrote:
>>>>>>>>>> The driver crc32c-intel match CPUs supporting
>>>>> X86_FEATURE_XMM4_2.
>>>>>>>>>> On platforms with Zhaoxin CPUs supporting this X86 feature,
>>>> When
>>>>>>>>>> crc32c-intel and crc32c-generic are both registered, system
>>>> will
>>>>>>>>>> use crc32c-intel because its .cra_priority is greater than
>>>>>>>>>> crc32c-generic. This case expect to use crc32c-generic driver
>>>>> for
>>>>>>>>>> some Zhaoxin CPUs to get performance gain, So remove these
>>>>> Zhaoxin
>>>>>>>>>> CPUs support from crc32c-intel.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Tony W Wang-oc <TonyWWang-oc@...oxin.com>
>>>>>>>>>
>>>>>>>>> Does this mean that the performance of the crc32c instruction
>>> on
>>>>> those CPUs is
>>>>>>>>> actually slower than a regular C implementation?  That's very
>>>>> weird.
>>>>>>>>>
>>>>>>>>
>>>>>>>> From the lmbench3 Create and Delete file test on those chips, I
>>>>> think yes.
>>>>>>>>
>>>>>>>
>>>>>>> Did you try measuring the performance of the hashing itself, and
>>>>> not some
>>>>>>> higher-level filesystem operations?
>>>>>>>
>>>>>>
>>>>>> Yes. Was testing on these Zhaoxin CPUs, the result is that with
>> the
>>>>> same
>>>>>> input value the generic C implementation takes fewer time than the
>>>>>> crc32c instruction implementation.
>>>>>>
>>>>>
>>>>> And that is really "working as intended"?
>>>>
>>>> These CPU's crc32c instruction is not working as intended.
>>>>
>>>>  Why do these CPUs even
>>>>> declare that
>>>>> they support the crc32c instruction, when it is so slow?
>>>>>
>>>>
>>>> The presence of crc32c and some other instructions supports are
>>>> enumerated by CPUID.01:ECX[SSE4.2] = 1,  other instructions are ok
>>>> except the crc32c instruction.
>>>>
>>>>> Are there any other instruction sets (AES-NI, PCLMUL, SSE, SSE2,
>> AVX,
>>>>> etc.) that
>>>>> these CPUs similarly declare support for but they are uselessly
>> slow?
>>>>
>>>> No.
>>>>
>>>> Sincerely
>>>> Tonyw
>>>>
>>>>>
>>>>> - Eric
>>>
>>> Then the right thing to do is to disable the CPUID bit in the
>>> vendor-specific startup code.
>>
>> This way makes these CPUs do not support all instruction sets
>> enumerated
>> by CPUID.01:ECX[SSE4.2].
>> While only crc32c instruction is slow, just expect the crc32c-intel
>> driver do not
>> match these CPUs.
>>
>> Sincerely
>> Tonyw
> 
> Then create a BUG flag for it, or factor out CRC32C into a synthetic flag. We *do not* bury this information in drivers; it becomes a recipe for the same problems over and over.
> 

Thanks for your suggestion. Have send new patch set.

Sincerely
Tonyw