Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Apr 28, 2015 at 3:54 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> I had a totally different implementation idea in mind.  It goes
> something like this:
>
> For each CPU, we allocate a fixed number of PCIDs, e.g. 0-7.  We have
> a per-cpu array of the mm [1] that owns each PCID. [...]

We've done this before on other architectures.  See for example alpha.
Look up "__get_new_mm_context()" and friends. I think sparc does the
same (and I think sparc copied a lot of it from the alpha
implementation).

Iirc, the alpha version just generates a (per-cpu) asid one at a time,
and has a generation counter so that when you run out of ASID's you do
a global TLB invalidate on that CPU and start from 0 again. Actually,
I think the generation number is just the high bits of the asid
counter (alpha calls them "asn", intel calls them "pcid", and I tend
to prefer "asid", but it's all the same thing).

Then each thread just has a per-thread ASID. We don't try to make that
be per-thread and per-cpu, but instead just force a new allocation
when a thread moves to another CPU.

It's not obvious what alpha does, because we end up hiding the
per-thread ASN in the "struct pcb_struct" (in 'struct thread_info')
which is part the alpha pal-code interface. But it seemed to work and
is fairly simple.

I think something very similar should work with intel pcid's.

                        Linus

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