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

[Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>]

On 05/14/2010 11:35 AM, Keir Fraser wrote:
> On 14/05/2010 10:22, "Joanna Rutkowska" <joanna@xxxxxxxxxxxxxxxxxxxxxx>
> wrote:
> 
>> 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.
> 
Hmmm... Shouldn't this affect only (and potentially) the text mode
display? I would expect that once Dom0 Linux takes over, it would be
using its own IGD driver that is VT-d aware and is not on the mercy of
the evil BIOS?

And we can disable the (ugly) Xen debug output, and have Dom0 linux boot
with kmodeset boot argument that would load the proper driver and switch
to a graphics mode right at the beginning.

j.

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