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

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Message-ID: <20190624102209.ngwtosgr5fvp3ler@willie-the-truck>
Date:   Mon, 24 Jun 2019 11:22:10 +0100
From:   Will Deacon <will@...nel.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>, julien.thierry@....com,
        aou@...s.berkeley.edu, james.morse@....com, suzuki.poulose@....com,
        Marc Zyngier <marc.zyngier@....com>,
        Anup Patel <anup.Patel@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, rppt@...ux.ibm.com,
        hch@...radead.org, Atish Patra <Atish.Patra@....com>,
        Julien Grall <julien.grall@....com>,
        Palmer Dabbelt <palmer@...ive.com>, gary@...yguo.net,
        paul.walmsley@...ive.com, christoffer.dall@....com,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
 separate file

On Mon, Jun 24, 2019 at 12:35:35AM +0800, Guo Ren wrote:
> On Fri, Jun 21, 2019 at 10:16 PM Catalin Marinas
> <catalin.marinas@....com> wrote:
> >
> > On Wed, Jun 19, 2019 at 07:51:03PM +0800, Guo Ren wrote:
> > > On Wed, Jun 19, 2019 at 4:54 PM Julien Grall <julien.grall@....com> wrote:
> > > > On 6/19/19 9:07 AM, Guo Ren wrote:
> > > > > 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?
> > >
> > > Almost done, but one question:
> > > arm64 remove the code in switch_mm:
> > >   cpumask_clear_cpu(cpu, mm_cpumask(prev));
> > >   cpumask_set_cpu(cpu, mm_cpumask(next));
> > >
> > > Why? Although arm64 cache operations could affect all harts with CTC
> > > method of interconnect, I think we should keep these code for
> > > primitive integrity in linux. Because cpu_bitmap is in mm_struct
> > > instead of mm->context.
> >
> > We didn't have a use for this in the arm64 code, so no point in
> > maintaining the mm_cpumask. On some arm32 systems (ARMv6) with no
> > hardware broadcast of some TLB/cache operations, we use it to track
> > where the task has run to issue IPI for TLB invalidation or some
> > deferred I-cache invalidation.
> The operation of set/clear mm_cpumask was removed in arm64 compared to
> arm32. It seems no side effect on current arm64 system, but from
> software meaning it's wrong.
> I think we should keep mm_cpumask just like arm32.

It was a while ago now, but I remember the atomic update of the mm_cpumask
being quite expensive when I was profiling this stuff, so I removed it
because we don't need it for arm64 (at least, it doesn't allow us to
optimise our shootdowns in practice).

I still think this is over-engineered for what you want on c-sky and making
this code generic is a mistake.

Will