linux-kernel - Re: [Intel-gfx] [PATCH v2] iommu/intel: Exclude devices using RMRRs from IOMMU API domains

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Message-ID: <1402989691.7595.110.camel@i7.infradead.org>
Date:	Tue, 17 Jun 2014 08:21:31 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Daniel Vetter <daniel@...ll.ch>
Cc:	Alex Williamson <alex.williamson@...hat.com>,
	iommu@...ts.linux-foundation.org, chegu_vinod@...com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>
Subject: Re: [Intel-gfx] [PATCH v2] iommu/intel: Exclude devices using RMRRs
 from IOMMU API domains

On Tue, 2014-06-17 at 09:15 +0200, Daniel Vetter wrote:
> We've always been struggling with stolen handling, and we've' always
> been struggling with vt-d stuff. Also pass-through seems to be a major
> pain (I've never tried myself). Given all that I'm voting for keeping
> the RMRR and everything else as much like for the normal case since I
> have no idea what exactly must be remapped and what's optional. The
> gpu is definitely keeping a lot of it's own private stuff in various
> chunks of stolen memory.

Keeping it like the normal case is distinctly non-trivial. I raised that
possibility, and it's hard. You have to make the guests' address maps
match the host, in that the E820-reserved regions used for DMA and
listed in RMRRs must also appear as reserved for the guests.

That was bad enough when it was just 'BIOS might be doing something evil
behind our back' and we didn't need to let the guest *access* those
pages. But in the i915 case we do actually map and access the stolen
region too, so the task is even harder. We'd need to be able to decide
when those regions should actually be mapped into the guest.

-- 
dwmw2

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