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

From: Catalin Marinas <[email-masked]>
Date: Fri, 21 Jun 2019 15:16:07 +0100

On Wed, Jun 19, 2019 at 07:51:03PM +0800, Guo Ren wrote:
> On Wed, Jun 19, 2019 at 4:54 PM Julien Grall <[email-masked]> 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/[email-masked]/
> > >
> > > 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.

(there was also a potential optimisation on arm64 to avoid broadcast
TLBI if the task only ran on a single CPU but Will found that was rarely
the case on an SMP system because of rebalancing happening during
execve(), ending up with two bits set in the mm_cpumask)

The way you use it on csky is different from how it is done on arm. It
seems to clear the mask for the scheduled out (prev) task but this
wouldn't work on arm(64) since the TLB still contains prev entries
tagged with the scheduled out ASID. Whether it matters, I guess it
depends on the specifics of your hardware.

While the algorithm may seem fairly generic, the semantics have a few
corner cases specific to each architecture. See [1] for a description of
the semantics we need on arm64 (CnP is a feature where the hardware
threads of the same core can share the TLB; the original algorithm
violated the requirements when this feature was enabled).

BTW, if you find the algorithm fairly straightforward ;), see this
bug-fix which took a formal model to identify: a8ffaaa060b8 ("arm64:
asid: Do not replace active_asids if already 0").

[1] https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/tree/asidalloc.tla#n79

-- 
Catalin

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