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

[Will Deacon]

On Sun, Sep 15, 2019 at 05:03:38AM +0000, Anup Patel wrote:
> 
> 
> > -----Original Message-----
> > From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-
> > owner@xxxxxxxxxxxxxxx> On Behalf Of Palmer Dabbelt
> > Sent: Saturday, September 14, 2019 7:31 PM
> > To: will@xxxxxxxxxx
> > Cc: guoren@xxxxxxxxxx; Will Deacon <will.deacon@xxxxxxx>;
> > julien.thierry@xxxxxxx; aou@xxxxxxxxxxxxxxxxx; james.morse@xxxxxxx;
> > Arnd Bergmann <arnd@xxxxxxxx>; suzuki.poulose@xxxxxxx;
> > marc.zyngier@xxxxxxx; catalin.marinas@xxxxxxx; Anup Patel
> > <Anup.Patel@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> > rppt@xxxxxxxxxxxxx; Christoph Hellwig <hch@xxxxxxxxxxxxx>; Atish Patra
> > <Atish.Patra@xxxxxxx>; julien.grall@xxxxxxx; gary@xxxxxxxxxxx; Paul
> > Walmsley <paul.walmsley@xxxxxxxxxx>; christoffer.dall@xxxxxxx; linux-
> > riscv@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
> > separate file
> > 
> > On Thu, 12 Sep 2019 07:02:56 PDT (-0700), will@xxxxxxxxxx wrote:
> > > 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@xxxxxxxxxx> 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.bruck
> > >> > er@xxxxxxx
> > >>
> > >> 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.
> > 
> > Ya, we're a long way from figuring this out.
> 
> For everyone's reference, here is our first attempt at RISC-V ASID allocator:
> http://archive.lwn.net:8080/linux-kernel/20190329045111.14040-1-anup.patel@xxxxxxx/T/#u

With a reply stating that the patch "absolutely does not work" ;)

What exactly do you want people to do with that? It's an awful lot of effort
to review this sort of stuff and given that Guo Ren is talking about sharing
page tables between the CPU and an accelerator, maybe you're better off
stabilising Linux for the platforms that you can actually test rather than
getting so far ahead of yourselves that you end up with a bunch of wasted
work on patches that probably won't get merged any time soon.

Seriously, they say "walk before you can run", but this is more "crawl
before you can fly". What's the rush?

Will