linux-kernel - Re: [PATCH v4 15/16] locking/rwsem: Merge owner into count on x86-64

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Message-ID: <20190418142822.GC12232@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Apr 2019 16:28:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 15/16] locking/rwsem: Merge owner into count on x86-64

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.