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

Arnd Bergmann
2014-05-21T16:01:06Z

On Wednesday 21 May 2014 08:44:42 Grant Grundler wrote:
> On Wed, May 21, 2014 at 2:32 AM, Arnd Bergmann <arnd@arndb.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