linux-kernel - Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

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Message-ID: <20190619091219.GB7767@fuggles.cambridge.arm.com>
Date:   Wed, 19 Jun 2019 10:12:19 +0100
From:   Will Deacon <will.deacon@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     Guo Ren <guoren@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        aou@...s.berkeley.edu, gary@...yguo.net, Atish.Patra@....com,
        hch@...radead.org, paul.walmsley@...ive.com, rppt@...ux.ibm.com,
        linux-riscv@...ts.infradead.org, Anup Patel <anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>, suzuki.poulose@....com,
        Marc Zyngier <marc.zyngier@....com>, catalin.marinas@....com,
        julien.thierry@....com, christoffer.dall@....com,
        james.morse@....com
Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
 separate file

On Wed, Jun 19, 2019 at 09:54:21AM +0100, Julien Grall wrote:
> On 6/19/19 9:07 AM, Guo Ren wrote:
> > You forgot CCing C-SKY folks :P
> 
> I wasn't aware you could be interested :).
> 
> > Move arm asid allocator code in a generic one is a agood idea, I've
> > made a patchset for C-SKY and test is on processing, See:
> > https://lore.kernel.org/linux-csky/1560930553-26502-1-git-send-email-guoren@kernel.org/
> > 
> > If you plan to seperate it into generic one, I could co-work with you.
> 
> Was the ASID allocator work out of box on C-Sky? If so, I can easily move
> the code in a generic place (maybe lib/asid.c).

This is one place where I'd actually prefer not to go down the route of
making the code generic. Context-switching and low-level TLB management
is deeply architecture-specific and I worry that by trying to make this
code common, we run the real risk of introducing subtle bugs on some
architecture every time it is changed. Furthermore, the algorithm we use
on arm64 is designed to scale to large systems using DVM and may well be
too complex and/or sub-optimal for architectures with different system
topologies or TLB invalidation mechanisms.

It's not a lot of code, so I don't see that it's a big deal to keep it
under arch/arm64.

Will