linux-kernel - Re: icache_is_aliasing and big.LITTLE

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Message-ID: <20190510110418.GA51370@lakrids.cambridge.arm.com>
Date:   Fri, 10 May 2019 12:04:18 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Salman Qazi <sqazi@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: icache_is_aliasing and big.LITTLE

On Thu, May 09, 2019 at 09:50:04AM +0100, Catalin Marinas wrote:
> Hi,
> 
> On Wed, May 08, 2019 at 11:45:03AM -0700, Salman Qazi wrote:
> > What is the intention behind icache_is_aliasing on big.LITTLE systems
> > where some icaches are VIPT and others are PIPT? Is it meant to be
> > conservative in some sense or should it be made per-CPU?
> 
> It needs to cover the worst case scenario across all CPUs, i.e. aliasing
> VIPT if one of the CPUs has this. We can't make it per-CPU because a
> thread performing cache maintenance might be migrated to another CPU
> with different cache policy (e.g. sync_icache_aliases()).

It's slightly more subtle than that -- for broadcast maintenance the
policy of the CPU receiving the broadcast matters.

So even if all i-cache maintenance were performed on a thread pinned to
a CPU with PIPT caches, to correctly affect any VIPT i-caches in the
system it would be necessary to perform maintenance as-if the CPU
performing the maintenance had VIPT i-caches.

Thanks,
Mark.