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Re: [Xen-devel] feature suggestion: DMAR table emulation for Xen

On 05/14/2010 07:41 AM, Kay, Allen M wrote:
> Joanna,
> RMRR memory ranges specified memory ranges that might be used for
> BIOS initiated DMA's for USB and integrated graphics devices.  These
> ranges are set by the BIOS.
> Do they stay the same across different platforms?

To be frank, those RMRR ranges have always been a bit of a mystery for
me. Do you actually know any OS drivers for USB devices that would
depend on them? Aren't they only used by the BIOS to implement various
dirty tricks they like to implement?

The Intel VT-d spec says that: "USB controllers and UMA integrated
graphics devices are the only legacy device usages identified
that depend on DMA to reserved system memory".

I think we can safely ignore the legacy integrated graphics devices,
because I would expect any VT-d enabled platform to have a modern
graphics device and modern OS drivers that works well with VT-d (you
even provide a dedicated DMA Remapping Unit for your integrated graphics
device, so I would expect it doesn't require any RMMR exceptions).

So USB devices seem to be the only potential problem. But USB devices
are implemented by the ICH, which in most cases is Intel ICH9, or 10.
So, if we took a DMAR from a platform, say PM45+ICH9M w/ Intel BIOS, it
should also work fine on any other laptop with this chipset (and
probably other variants too), just minus the potential


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