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

To: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] feature suggestion: DMAR table emulation for Xen
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 14 May 2010 11:48:33 +0100
Cc: "Han, Weidong" <weidong.han@xxxxxxxxx>, "Cihula, Joseph" <joseph.cihula@xxxxxxxxx>, "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 14/05/2010 11:15, "Joanna Rutkowska" <joanna@xxxxxxxxxxxxxxxxxxxxxx>

>> 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?

Well, if you do not pass through the IGD to a domU then the issue is moot.
Dom0 gets an all-inclusive mapping below 4GB, which should be a superset of
anything the RMRRs would specify. It's when passing through to a domU that
the RMRRs matter, especially if you pass through as the primary adaptor and
hence re-execute the video BIOS in domU context.

 -- Keir

> 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.

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