Re: IOMMU and graphics cards

Joerg Roedel
2009-04-28T16:05:29Z

On Tue, Apr 28, 2009 at 04:28:50PM +0100, David Woodhouse wrote:
> On Tue, 2009-04-28 at 17:05 +0200, Joerg Roedel wrote:
>
> For that device, yes. The IOMMU still catches errors from _other_
> devices, of course.

Yes, it is in effect for other devices. But since its a security feature
it only makes sense if it covers all devices.

> Is the reason you're doing this actually because of broken drivers? Or
> just because of the performance implications? I've heard both cited as
> reasons for the 1:1 mapping... and if it's mostly the former, then
> perhaps the best way for me to help you is to stop enabling the option
> in Fedora (rawhide, at least), so that the buggy drivers get _fixed_ and
> you don't get stuck with "it works on Intel, why can't you make it
> work?" reports?

Currently some graphics drivers don't work with IOMMU enabled because
they don't use the DMA-API. The silently assume that device addess ==
physical address.
I don't know how it is solved for VT-d but for AMD IOMMU the available
DMA memory address space is limited per device. This is a problem for
graphics card drivers because, as developers told me, they may need
gigabytes of DMA memory. I am currently thinking about ways to fix that
without enlarging the DMA address space for each device (which would be
a huge waste of memory). But unless this problem isn't solved the
drivers won't be fixed, I guess.
I guess the DRM code in the kernel may have the same problem with IOMMU
enabled?

> > * Implement a kernel command line option to enable/disable the
> > workaround (what should be default?)
> > * Use the IOMMU-API and write a kernel module which creates a
> > direct mapped protection domain and assigns the graphics
> > cards to it (need to be done carefully to not break graphics
> > drivers which do everything right and use the DMA-API)
> > * Any other great idea?
>
> I think I also prefer your second option. I don't like having this as a
> hack in the IOMMU code.

Ok, so I will implement such a module and send the code around for
discussion.

Joerg

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