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Subject: Re: [PATCH v4 15/16] locking/rwsem: Merge owner into count on x86-64
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
References: <20190413172259.2740-1-longman@redhat.com>
 <20190413172259.2740-16-longman@redhat.com>
 <20190418142822.GC12232@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
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Message-ID: <a70ee6c0-7904-8c63-e956-57f6d930db07@redhat.com>
Date:   Thu, 18 Apr 2019 10:40:04 -0400
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On 04/18/2019 10:28 AM, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:58PM -0400, Waiman Long wrote:
>> +#ifdef CONFIG_X86_64
>> +#define RWSEM_MERGE_OWNER_TO_COUNT
>> +#endif
>> +#ifdef RWSEM_MERGE_OWNER_TO_COUNT
>> +
>> +#ifdef __PHYSICAL_MASK_SHIFT
>> +#define RWSEM_PA_MASK_SHIFT	__PHYSICAL_MASK_SHIFT
>> +#else
>> +#define RWSEM_PA_MASK_SHIFT	52
>> +#endif
> I really dislike how this hardcodes x86_64.
>
> It would be much better to have a CONFIG variable that gives us the
> PA_BITS. Then all an architecture needs to do is set that right.
>
> FWIW:
>
> arch/arm64/Kconfig:config ARM64_PA_BITS_48
> arch/arm64/Kconfig:config ARM64_PA_BITS_52
>
> So ARM64 could also use this -- provided we get that overflow thing
> sorted.

That will work too. I just a bit hesitant to add new config variable.

Cheers,
Longman