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

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Date:	Thu, 24 Jul 2014 16:33:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Joerg Roedel <joro@...tes.org>
Cc:	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] mmu_notifier: Allow to manage CPU external TLBs

On Thu, 24 Jul 2014 16:35:38 +0200 Joerg Roedel <joro@...tes.org> wrote:

> 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.
> 
> 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!

It looks pretty simple and harmless.

I assume the AMD IOMMUv2 driver actually uses this and it's all
tested and good?  What is the status of that driver?

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