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

On 14/05/2010 10:22, "Joanna Rutkowska" <joanna@xxxxxxxxxxxxxxxxxxxxxx>

> 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).

Yeah, actually the integrated graphics can implement all sorts of dirty
tricks between OS driver, video BIOS, and SMM. This can rely on fixed memory
areas for communication -- both for host accesses and DMA, the latter
requiring RMRR setup. Maybe the RMRRs are static per-chipset, but I wouldn't
be too sure of it.

 -- Keir

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