Re: [PATCH 08/20] mm: Optimize fullmm TLB flushing

[Peter Zijlstra]

On Thu, 2012-06-28 at 13:19 +0200, Martin Schwidefsky wrote:

> The cpu can create speculative TLB entries, but only if it runs in the
> mode that uses the respective mm. We have two mm's active at the same
> time, the kernel mm (init_mm) and the user mm. While the cpu runs only
> in kernel mode it is not allowed to create TLBs for the user mm.
> While running in user mode it is allowed to speculatively create TLBs.

OK, that's neat.

> Basically we have two special requirements on s390:
> 1) do not modify ptes while attached to another cpu except with the
>    special IPTE / IDTE instructions

Right, and your fullmm case works by doing a global invalidate after all
threads have ceased userspace execution, this allows you to do away with
the IPTE/IDTE instructions since there's no other active cpus on the
userspace mm anymore.


> 2) do a TLB flush before freeing any kind of page table page, s390
>    needs a flush for pud, pmd & pte tables. 

Right, we do that (now)..
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