linux-kernel - Re: [PATCH 0/3 v2] mmu_notifier: Allow to manage CPU external TLBs

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Date:	Thu, 31 Jul 2014 10:54:16 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Jerome Glisse <jglisse@...hat.com>, jroedel@...e.de,
	Jay.Cornwall@....com, Oded.Gabbay@....com, John.Bridgman@....com,
	Suravee.Suthikulpanit@....com, ben.sander@....com,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 0/3 v2] mmu_notifier: Allow to manage CPU external TLBs

On Tue, Jul 29, 2014 at 06:18:10PM +0200, Joerg Roedel wrote:
> Changes V1->V2:
> 
> * Rebase to v3.16-rc7
> * Added call of ->invalidate_range to
>   __mmu_notifier_invalidate_end() so that the subsystem
>   doesn't need to register an ->invalidate_end() call-back,
>   subsystems will likely either register
>   invalidate_range_start/end or invalidate_range, so that
>   should be fine.
> * Re-orded declarations a bit to reflect that
>   invalidate_range is not only called between
>   invalidate_range_start/end
> * Updated documentation to cover the case where
>   invalidate_range is called outside of
>   invalidate_range_start/end to flush page-table pages out
>   of the TLB
> 
> Hi,
> 
> here is a patch-set to extend the mmu_notifiers in the Linux
> kernel to allow managing CPU external TLBs. Those TLBs may
> be implemented in IOMMUs or any other external device, e.g.
> ATS/PRI capable PCI devices.
> 
> The problem with managing these TLBs are the semantics of
> the invalidate_range_start/end call-backs currently
> available. Currently the subsystem using mmu_notifiers has
> to guarantee that no new TLB entries are established between
> invalidate_range_start/end. Furthermore the
> invalidate_range_start() function is called when all pages
> are still mapped and invalidate_range_end() when the pages
> are unmapped an already freed.
> 
> So both call-backs can't be used to safely flush any non-CPU
> TLB because _start() is called too early and _end() too
> late.
> 
> In the AMD IOMMUv2 driver this is currently implemented by
> assigning an empty page-table to the external device between
> _start() and _end(). But as tests have shown this doesn't
> work as external devices don't re-fault infinitly but enter
> a failure state after some time.
> 
> Next problem with this solution is that it causes an
> interrupt storm for IO page faults to be handled when an
> empty page-table is assigned.
> 
> Furthermore the _start()/end() notifiers only catch the
> moment when page mappings are released, but not page-table
> pages. But this is necessary for managing external TLBs when
> the page-table is shared with the CPU.
> 
> To solve this situation I wrote a patch-set to introduce a
> new notifier call-back: mmu_notifer_invalidate_range(). This
> notifier lifts the strict requirements that no new
> references are taken in the range between _start() and
> _end(). When the subsystem can't guarantee that any new
> references are taken is has to provide the
> invalidate_range() call-back to clear any new references in
> there.
> 
> It is called between invalidate_range_start() and _end()
> every time the VMM has to wipe out any references to a
> couple of pages. This are usually the places where the CPU
> TLBs are flushed too and where its important that this
> happens before invalidate_range_end() is called.
> 
> Any comments and review appreciated!

For the series :

Reviewed-by: Jérôme Glisse <jglisse@...hat.com>

> 
> Thanks,
> 
> 	Joerg
> 
> Joerg Roedel (3):
>   mmu_notifier: Add mmu_notifier_invalidate_range()
>   mmu_notifier: Call mmu_notifier_invalidate_range() from VMM
>   mmu_notifier: Add the call-back for mmu_notifier_invalidate_range()
> 
>  include/linux/mmu_notifier.h | 75 +++++++++++++++++++++++++++++++++++++++++---
>  kernel/events/uprobes.c      |  2 +-
>  mm/fremap.c                  |  2 +-
>  mm/huge_memory.c             |  9 +++---
>  mm/hugetlb.c                 |  7 ++++-
>  mm/ksm.c                     |  4 +--
>  mm/memory.c                  |  3 +-
>  mm/migrate.c                 |  3 +-
>  mm/mmu_notifier.c            | 25 +++++++++++++++
>  mm/rmap.c                    |  2 +-
>  10 files changed, 115 insertions(+), 17 deletions(-)
> 
> -- 
> 1.9.1
> 
> --
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