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

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Date:	Fri, 25 Jul 2014 03:10:28 +0000
From:	"Sander, Ben" <ben.sander@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Joerg Roedel <joro@...tes.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" <jroedel@...e.de>,
	"Cornwall, Jay" <Jay.Cornwall@....com>,
	"Gabbay, Oded" <Oded.Gabbay@....com>,
	"Bridgman, John" <John.Bridgman@....com>,
	"Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH 0/3] mmu_notifier: Allow to manage CPU external TLBs

Yes. AMD has tested this with the iommuv2 driver and verified it works correctly.  There is a corresponding change in the iommuv2 driver to use the new API. 

> On Jul 24, 2014, at 6:33 PM, "Andrew Morton" <akpm@...ux-foundation.org> wrote:
> 
>> 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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