Re: IOMMU and graphics cards

Ingo Molnar
2009-05-07T10:56:19Z

* Joerg Roedel <joerg.roedel@amd.com> wrote:

> On Thu, May 07, 2009 at 04:47:14PM +0900, FUJITA Tomonori wrote:
> > On Tue, 28 Apr 2009 18:04:48 +0200
> > Joerg Roedel <joerg.roedel@amd.com> wrote:
> > >
> > > Yes, it is in effect for other devices. But since its a security feature
> > > it only makes sense if it covers all devices.
> >
> > For me, IOMMUs are not about security but accessing all memory space
> > (there are still lots of devices that are not capable of 64-bit DMA)
> > but I agree that this workaround is bad.
>
> Yeah true. Another major reason for having an IOMMU is to enlarge the
> physical address range devices can access.
>
> > Do you meant that AMD IOMMU code use the bitmap each device to manage
> > address space so enlarging the DMA address space wastes memory?
>
> Its not only the bitmap. There are also the page tables that would waste
> memory if the aperture in the AMD IOMMU driver is enlarged to, say, 4GB.
> Most devices don't need such a large amount of DMA address space. As I
> found out VT-d does not have this problem because they implemented
> another allocator which can cover all possible memory.
> For the AMD IOMMU I currently implement a kind of a dynamically growing
> bitmap allocator to fix this problem.
>
> > > 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?
> >
> > Looks like the DRM code uses the DMA API.
>
> The proprietary drivers make problems so far. For the ATI one I am
> in contact with the developers to try to fix it. But I can't do
> anything about the other proprietary driver I am aware of :-(

One option would be to enumerate nvidia vga device memory resources,
print a nasty warning if they are accessed while not mapped by any
in-kernel driver (such as Noveau) and then install a workaround for
their resources and exclude them from IOMMU coverage.

Ingo