Re: [GIT PULL] AMD IOMMU updates for 2.6.28-rc5

Ingo Molnar
2008-11-20T07:52:18Z

* FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:

> > > It had been the default option for AMD IOMMU until you hit the
> > > bugs. IIRC, VT-d also shares a protection domain by default. It
> > > would be nice to avoid surprising users if the two
> > > virtualization IOMMUs works in the similar way.
> >
> > Calgary has a per-bus protection domain, both on x86 and PPC.
>
> I see. Then it might be better to change VT-d to use a separate
> protection domain by default.

yes, agreed, and that should be the sane default for any IOMMU driver
- unless the performance impact is prohibitive.

Note that this widens the positive impact of the IOMMU code: not only
does it enable transparent support of DMA to/from devices that have a
limited DMA range, not only does it help isolation in virtualization -
it also acts as a daily debug helper for _native_ drivers.

Note that people will prefer to run with an IOMMU enabled even if all
devices support the full memory range - just due to the DMA protection
features. Just like people prefer to run an OS with paging protections
enabled ;-)

It also puts pressure on the hw design side to treat IOMMUs not just
as some fringe feature for compatibility with older transports or
virtualization, but also as a prime-time native IO feature.

Ingo