linux-kernel - Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

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Date:	Wed, 21 May 2014 18:01:06 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Grant Grundler <grundler@...omium.org>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	Linux DeviceTree <devicetree@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Joerg Roedel <joro@...tes.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>, linux-tegra@...r.kernel.org,
	Cho KyongHo <pullip.cho@...sung.com>,
	Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

On Wednesday 21 May 2014 08:44:42 Grant Grundler wrote:
> On Wed, May 21, 2014 at 2:32 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Wednesday 21 May 2014 11:02:45 Thierry Reding wrote:
> >> On Wed, May 21, 2014 at 10:54:42AM +0200, Arnd Bergmann wrote:
> >>
> >> > Right. As long as we always unmap the buffers from the IOMMU after they
> >> > have stopped being in use, it's very unlikely that even a broken device
> >> > driver causes a DMA into some bus address that happens to be mapped for
> >> > another device.
> >>
> >> I think that if buffers remain mapped in the IOMMU when they have been
> >> deallocated that should be considered a bug.
> 
> There is currently no general requirement to tear down mappings immediately.
> An option to enforce immediate tear down might be useful since I agree
> Virtual Guests sharing host memory through shared physical devices
> will want that. ie there should be no opportunity for a shared device
> to be able to access another guests memory through "stale" (but live)
> DMA mappings. I don't think that's the case but I'd ask someone like
> Alex Williamson for a more certain answer.

I believe powerpc has a boot-time option to enforce the immediate
IOTLB flush, which is very useful for device driver debugging when
something goes wrong with stale DMAs.

> > Obviously you have to maintain the IO page tables correctly.
> 
> To be clear, "Correctly" in this case just means until the IOTLB is
> flushed, the given IOMMU Pdir entries are marked "in use" even though
> the driver has handed "ownership" back to the IOMMU driver.

I don't know what a Pdir is, but I guess strictly speaking we have
to ensure that all IO page table entries that have been unmapped by
a driver are marked as invalid at least by the time the IOTLB is flushed,
plus we have to flush each entry before it gets reused for a different
page.

	Arnd
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