linux-kernel - Re: [RFC PATCH 01/10] arm64: feature registers: Documentation

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Date:	Tue, 11 Aug 2015 09:41:01 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Andrew Haley <aph@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"edward.nevill@...aro.org" <edward.nevill@...aro.org>
Subject: Re: [RFC PATCH 01/10] arm64: feature registers: Documentation

On 10/08/15 19:19, Andrew Haley wrote:
> On 08/10/2015 06:36 PM, Suzuki K. Poulose wrote:
>> On 10/08/15 17:06, Catalin Marinas wrote:
>>> Hi Suzuki,
>>>
>>> On Fri, Jul 24, 2015 at 10:43:47AM +0100, Suzuki K. Poulose wrote:
>>>> From: "Suzuki K. Poulose" <suzuki.poulose@....com>
>>>>
>>>> Documentation of the infrastructure
>>>>
>>>> Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com>
>>>
>>> The implementation looks fine but I think the main discussion will be
>>> around the goal of this feature and the ABI that it introduces. So I'll
>>> just write my thoughts on this patch (I could as well have replied to
>>> the cover letter).
>>>
>>> Another question: who's going to use this feature? I know people asked
>>> in private but I'd like to have some public statements.
>>
>> Right, I am hoping that folks from glibc / JIT / GCC will respond to
>> this thread.
>
> We certainly need it for OpenJDK.  We need to know the manufacturer,
> part number, revision id, etc.  We already have workarounds in
> OpenJDK for various bugs, and we also can generate better code if we
> know the exact part.

OK.

>
> I note that the REVIDR is not in this patch.  That seems odd, because
> it can be used to identify minor revisions.

The REVIDR has to be used in conjunction with the MIDR to make real sense.
We cannot guarantee that the REVIDR that we read (would) belong to the CPU
where MIDR would have been read (unless the process is pinned) and hence the
user may not be able to make any use of the information. Steve has a patch [1]
to expose the MIDR,REVIDR info via sysfs.

[1] https://lkml.org/lkml/2015/7/24/420

Thanks
Suzuki

>
> Andrew.
>
>

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