From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>, linux-mm@kvack.org
Subject: Re: TLB flushes on s390
Date: Wed, 6 May 2015 14:56:40 +0200 [thread overview]
Message-ID: <20150506145640.70ee8686@mschwide> (raw)
In-Reply-To: <20150506125000.GB17739@node.dhcp.inet.fi>
On Wed, 6 May 2015 15:50:00 +0300
"Kirill A. Shutemov" <kirill@shutemov.name> wrote:
> On Wed, May 06, 2015 at 02:00:02PM +0200, Martin Schwidefsky wrote:
> > 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.
>
> Okay, it makes some sense.
>
> One more question: why does __tlb_flush_full()/__tlb_flush_asce() require
> disabling preemption and pmdp_flush_direct() doesn't?
Easy: for pmdp_flush_direct you are required to hold the pmd-lock as you
modify the pmd. The spinlock provides the preempt_disable.
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
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prev parent reply other threads:[~2015-05-06 12:56 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-06 11:29 TLB flushes on s390 Kirill A. Shutemov
2015-05-06 12:00 ` Martin Schwidefsky
2015-05-06 12:50 ` Kirill A. Shutemov
2015-05-06 12:56 ` Martin Schwidefsky [this message]
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