linux-kernel - Re: [PATCH v10 02/25] x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE

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Message-ID: <87o9hi46si.fsf@e105922-lin.cambridge.arm.com>
Date:   Mon, 14 May 2018 16:05:01 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, peterz@...radead.org,
        kirill@...temov.name, ak@...ux.intel.com, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v10 02/25] x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT

Laurent Dufour <ldufour@...ux.vnet.ibm.com> writes:

> On 08/05/2018 13:04, Punit Agrawal wrote:
>> Hi Laurent,
>> 
>> Laurent Dufour <ldufour@...ux.vnet.ibm.com> writes:
>> 
>>> Set ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT which turns on the
>>> Speculative Page Fault handler when building for 64bit.
>>>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
>>> ---
>>>  arch/x86/Kconfig | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index d8983df5a2bc..ebdeb48e4a4a 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -30,6 +30,7 @@ config X86_64
>>>  	select MODULES_USE_ELF_RELA
>>>  	select X86_DEV_DMA_OPS
>>>  	select ARCH_HAS_SYSCALL_WRAPPER
>>> +	select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
>> 
>> I'd suggest merging this patch with the one making changes to the
>> architectural fault handler towards the end of the series.
>> 
>> The Kconfig change is closely tied to the architectural support for SPF
>> and makes sense to be in a single patch.
>> 
>> If there's a good reason to keep them as separate patches, please move
>> the architecture Kconfig changes after the patch adding fault handler
>> changes.
>> 
>> It's better to enable the feature once the core infrastructure is merged
>> rather than at the beginning of the series to avoid potential bad
>> fallout from incomplete functionality during bisection.
>
> Indeed bisection was the reason why Andrew asked me to push the configuration
> enablement on top of the series (https://lkml.org/lkml/2017/10/10/1229).

The config options have gone through another round of splitting (between
core and architecture) since that comment. I agree that it still makes
sense to define the core config - CONFIG_SPECULATIVE_PAGE_FAULT early
on.

Just to clarify, my suggestion was to only move the architecture configs
further down.

>
> I also think it would be better to have the architecture enablement in on patch
> but that would mean that the code will not be build when bisecting without the
> latest patch adding the per architecture code.

I don't see that as a problem. But if I'm in the minority, I am OK with
leaving things as they are as well.

Thanks,
Punit