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Subject: Re: [PATCH V12] mm/debug: Add tests validating architecture page
 table helpers
To: Qian Cai <cai@lca.pw>, Anshuman Khandual <anshuman.khandual@arm.com>
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From: Christophe Leroy <christophe.leroy@c-s.fr>
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Date: Tue, 28 Jan 2020 07:13:04 +0100
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Cc: Mark Rutland <Mark.Rutland@arm.com>, linux-ia64@vger.kernel.org,
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Le 28/01/2020 à 04:33, Qian Cai a écrit :
> 
> 
>> On Jan 27, 2020, at 10:06 PM, Anshuman Khandual <anshuman.khandual@arm.com> wrote:
>>
>>
>>
>> On 01/28/2020 07:41 AM, Qian Cai wrote:
>>>
>>>
>>>> On Jan 27, 2020, at 8:28 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote:
>>>>
>>>> This adds tests which will validate architecture page table helpers and
>>>> other accessors in their compliance with expected generic MM semantics.
>>>> This will help various architectures in validating changes to existing
>>>> page table helpers or addition of new ones.
>>>>
>>>> This test covers basic page table entry transformations including but not
>>>> limited to old, young, dirty, clean, write, write protect etc at various
>>>> level along with populating intermediate entries with next page table page
>>>> and validating them.
>>>>
>>>> Test page table pages are allocated from system memory with required size
>>>> and alignments. The mapped pfns at page table levels are derived from a
>>>> real pfn representing a valid kernel text symbol. This test gets called
>>>> right after page_alloc_init_late().
>>>>
>>>> This gets build and run when CONFIG_DEBUG_VM_PGTABLE is selected along with
>>>> CONFIG_VM_DEBUG. Architectures willing to subscribe this test also need to
>>>> select CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE which for now is limited to x86 and
>>>> arm64. Going forward, other architectures too can enable this after fixing
>>>> build or runtime problems (if any) with their page table helpers.
>>
>> Hello Qian,
>>
>>>
>>> What’s the value of this block of new code? It only supports x86 and arm64
>>> which are supposed to be good now.
>>
>> We have been over the usefulness of this code many times before as the patch is
>> already in it's V12. Currently it is enabled on arm64, x86 (except PAE), arc and
>> ppc32. There are build time or runtime problems with other archs which prevent
> 
> I am not sure if I care too much about arc and ppc32 which are pretty much legacy
> platforms.
> 
>> enablement of this test (for the moment) but then the goal is to integrate all
>> of them going forward. The test not only validates platform's adherence to the
>> expected semantics from generic MM but also helps in keeping it that way during
>> code changes in future as well.
> 
> Another option maybe to get some decent arches on board first before merging this
> thing, so it have more changes to catch regressions for developers who might run this.
> 

ppc32 an indecent / legacy platform ? Are you kidying ?

Powerquicc II PRO for instance is fully supported by the manufacturer 
and widely used in many small networking devices.

Christophe