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

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Date:   Thu, 12 Sep 2019 15:02:56 +0100
From:   Will Deacon <will@...nel.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Will Deacon <will.deacon@....com>, julien.thierry@....com,
        aou@...s.berkeley.edu, james.morse@....com,
        Arnd Bergmann <arnd@...db.de>, suzuki.poulose@....com,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anup Patel <anup.Patel@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        Atish Patra <Atish.Patra@....com>,
        Julien Grall <julien.grall@....com>,
        Palmer Dabbelt <palmer@...ive.com>, gary@...yguo.net,
        Paul Walmsley <paul.walmsley@...ive.com>,
        christoffer.dall@....com, linux-riscv@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
 separate file

On Sun, Sep 08, 2019 at 07:52:55AM +0800, Guo Ren wrote:
> On Mon, Jun 24, 2019 at 6:40 PM Will Deacon <will@...nel.org> wrote:
> > > I'll keep my system use the same ASID for SMP + IOMMU :P
> >
> > You will want a separate allocator for that:
> >
> > https://lkml.kernel.org/r/20190610184714.6786-2-jean-philippe.brucker@arm.com
> 
> Yes, it is hard to maintain ASID between IOMMU and CPUMMU or different
> system, because it's difficult to synchronize the IO_ASID when the CPU
> ASID is rollover.
> But we could still use hardware broadcast TLB invalidation instruction
> to uniformly manage the ASID and IO_ASID, or OTHER_ASID in our IOMMU.

That's probably a bad idea, because you'll likely stall execution on the
CPU until the IOTLB has completed invalidation. In the case of ATS, I think
an endpoint ATC is permitted to take over a minute to respond. In reality, I
suspect the worst you'll ever see would be in the msec range, but that's
still an unacceptable period of time to hold a CPU.

> Welcome to join our disscusion:
> "Introduce an implementation of IOMMU in linux-riscv"
> 9 Sep 2019, 10:45 Jade-room-I&II (Corinthia Hotel Lisbon) RISC-V MC

I attended this session, but it unfortunately raised many more questions
than it answered.

Will