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

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Date:	Thu, 19 Jun 2014 08:10:00 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	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 Thu, Jun 19, 2014 at 3:47 AM, Alex Williamson
<alex.williamson@...hat.com> wrote:
> Finding some more specs... the MGGC0 register (50h) seems to indicate
> the GTT stolen memory size is 2M, which sounds suspiciously like the 2M
> that the RMRR is reporting.  However, from the IvyBridge MMIO, Media
> Registers & Programming Env manual:
>
>         4.6.1 Changes to GTT
>
>         The GTT is constrained to be located at the beginning of a
>         special section of stolen memory called the GTT stolen memory
>         (GSM). There is no longer an MMIO register containing the
>         physical base address of the GTT as on prior devices. Instead of
>         using the PGTBL_CTL register to specify the base address of the
>         GTT, the GTT base is now defined to be at the bottom (offset 0)
>         of GSM.
>
>         Since the graphics device (including the driver) knows nothing
>         about the location of GSM, it does not “know” where the GTT is
>         located in memory. In fact, the CPU cannot directly access the
>         GSM containing the GTT.
>
> That seems to suggest we can't discover this region from the device, but
> the device does need to maintain access to it... I don't know how to
> resolve that without exposing the RMRR through the IOMMU API.
>
> In any case, I don't know that any of this should block the original
> patch.  All of this seems like "acceptable" use of RMRRs that we can
> later add an exception to allow if we get to the point of understanding
> it and being able to reproduce any required mappings in the guest.
> Thanks,

GTT stolen is the place where the gpu stores page tables. We never
access them directly but through a special mmio range so that the gpu
can intercept pte updates and invalidate tlbs accordingly. So yeah, we
need this, too.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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