linux-kernel - Re: [RFC PATCH 1/6] arm64: use tpidr_el1 for current, free sp

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Message-ID: <20170714013054.GE22336@arm.com>
Date:   Fri, 14 Jul 2017 02:30:54 +0100
From:   Will Deacon <will.deacon@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     ard.biesheuvel@...aro.org, kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        akashi.takahiro@...aro.org, catalin.marinas@....com,
        dave.martin@....com, james.morse@....com,
        labbott@...oraproject.org, keescook@...omium.org
Subject: Re: [RFC PATCH 1/6] arm64: use tpidr_el1 for current, free sp_el0

On Wed, Jul 12, 2017 at 11:32:58PM +0100, Mark Rutland wrote:
> Today we use TPIDR_EL1 for our percpu offset, and SP_EL0 for current
> (and current::thread_info, which is at offset 0).
> 
> Using SP_EL0 in this way prevents us from using EL1 thread mode, where
> SP_EL0 is not addressable (since it's used as the active SP). It also
> means we can't use SP_EL0 for other purposes (e.g. as a
> scratch-register).
> 
> This patch frees up SP_EL0 for such usage, by storing the percpu offset
> in current::thread_info, and using TPIDR_EL1 to store current. As we no
> longer need to update SP_EL0 at EL0 exception boundaries, this allows us
> to delete some code.

Does this mean we can just use asm-generic/percpu.h?

Will