Re: TLB flushes on s390

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Wed, 6 May 2015 14:29:39 +0300
"Kirill A. Shutemov" <kirill@shutemov.name> wrote:

> I'm looking though s390 code around page table handling and I found that
> in many places s390 does tlb flush before changing page table entry.

Uhh, have fun with that.. it is complicated :-/
 
> Let's look for instance on pmdp_clear_flush() implementation on s390.
> It's implemented with pmdp_get_and_clear() which does pmdp_flush_direct()
> *before* pmd_clear(). That's invert order comparing to generic
> pmdp_flush_direct().
> 
> The question is what prevents tlb from being re-fill between flushing tlb
> and clearing page table entry?
 
Look again at pmdp_flush_direct(), either __pmdp_idte_local or __pmdp_idte is
called. Both functions use the IDTE instruction but in two different flavors.
The mnemonic IDTE stands for invalidate-dat-table-entry, the instruction sets
the invalid bit in the PMD and flushes all TLB entries on all CPUs that are
affected by the now invalid PMD. The pmd_clear after the pmdp_flush_direct is
done to set all the other bits of the PMD to the "empty" state. The invalid
bit is already set prior to pmd_clear.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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