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

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Message-ID: <1403008864.7595.144.camel@i7.infradead.org>
Date:	Tue, 17 Jun 2014 13:41:04 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	iommu@...ts.linux-foundation.org, chegu_vinod@...com,
	linux-kernel@...r.kernel.org,
	Intel Graphics Development <intel-gfx@...ts.freedesktop.org>
Subject: Re: [PATCH v2] iommu/intel: Exclude devices using RMRRs from IOMMU
 API domains

On Tue, 2014-06-17 at 06:22 -0600, Alex Williamson wrote:
> On Tue, 2014-06-17 at 08:04 +0100, David Woodhouse wrote:
> > On Mon, 2014-06-16 at 23:35 -0600, Alex Williamson wrote:
> > > 
> > > Any idea what an off-the-shelf Asus motherboard would be doing with an
> > > RMRR on the Intel HD graphics?
> > > 
> > > dmar: RMRR base: 0x000000bb800000 end: 0x000000bf9fffff
> > > IOMMU: Setting identity map for device 0000:00:02.0 [0xbb800000 - 0xbf9fffff]
> > 
> > Hm, we should have thought of that sooner. That's quite normal — it's
> > for the 'stolen' memory used for the framebuffer. And maybe also the
> > GTT, and shadow GTT and other things; I forget precisely what, and it
> > varies from one setup to another.
> 
> Why exactly do these things need to be identity mapped through the
> IOMMU?  This sounds like something a normal device might do with a
> coherent mapping.

The BIOS (EFI or VESA) sets up a framebuffer in stolen main memory. It's
accessed by DMA, using the physical address. The RMRR exists because we
need it *not* to suddenly stop working the moment the OS turns on the
IOMMU.

The OS graphics driver, if any, is not loaded at this point.

And even later, the OS graphics driver may choose to make use of the
'stolen' memory for various purposes. And since it was already stolen,
it doesn't go and set up *another* mapping for it; it knows that a
mapping already exists.

> > I'd expect fairly much all systems to have an RMRR for the integrated
> > graphics device if they have one, and your patch¹ is going to prevent
> > assignment of those to guests... as you've presumably noticed.
> > 
> > I'm not sure if the i915 driver is capable of fully reprogramming the
> > hardware to completely stop using that region, to allow assignment to a
> > guest with a 'pure' memory map and no stolen region. I suppose it must,
> > if assignment to guests was working correctly before?
> 
> IGD assignment has never worked with KVM.

Hm. It works with Xen though, doesn't it?

Are we content to say that it'll *never* work with KVM, and thus we can
live with the fact that your patch makes it harder to fix whatever was
wrong in the first place?

-- 
dwmw2

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