Re: VGA arbiter support for Intel HD?

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Aug 14, 2013 at 07:23:57AM -0600, Alex Williamson wrote:
> Hi,
> 
> I'm trying to add support for device assignment of PCI VGA devices with
> VFIO and QEMU.  For normal, discrete discrete graphics the Linux VGA
> arbiter works fairly well, disabling VGA on one bridge and adding it to
> another (though I wish all the kernel VGA drivers made use of it).  The
> i915 driver only seems to support disabling VGA on really old GMCH
> devices (see intel_modeset_vga_set_state).  This means that if I boot
> with IGD as the primary graphics and attempt to assign a discrete
> graphics device, all the VGA range accesses are still routed to IGD, I
> end up getting some error messages from the IGD interrupt handler, and
> the discrete card never initializes.
> 
> I spent some time looking through the Sand Bridge, Ivy Bridge, and
> Haswell datasheets, and I'm a bit concerned whether the hardware even
> provides a reasonable way to disable VGA anymore.  Quoting 2.17 from the
> Haswell docs:
> 
>         Accesses to the VGA memory range are directed to IGD depend on
>         the configuration.  The configuration is specified by:
>               * Internal graphics controller in Device 2 is enabled
>                 (DEVEN.D2EN bit 4)
>               * Internal graphics VGA in Device 0 Function 0 is enabled
>                 through register GGC bit 1.
>               * IGD's memory accesses (PCICMD2 04 – 05h, MAE bit 1) in
>                 Device 2 configuration space are enabled.
>               * VGA compatibility memory accesses (VGA Miscellaneous
>                 Output register – MSR Register, bit 1) are enabled.
>               * Software sets the proper value for VGA Memory Map Mode
>                 register (VGA GR06 Register, bits 3-2). See the
>                 following table for translations.
> 
> (There's a similar list for VGA I/O range)  I've found that if I disable
> memory and I/O in the PCI command register for IGD then I do get VGA
> routing to the PEG device and the discrete VBIOS works.  This obviously
> isn't a good option for the VGA arbiter since it entirely disables IGD.
> 
> The GGC registers aren't meant for runtime switching and are actually
> locked.  Disabling IGD via the device 2 enable bit doesn't seem like and
> option.  I don't quite understand the VGA miscellaneous output register
> and VGA memory map mode, but the table provided for the latter makes me
> think they just augment the VGA ranges and don't disable them.

Bit 1 of MSR (0x3c2/0x3cc) should allow you to turn off VGA mem
access while leaving other memory space access working.

As for VGA I/O decode, IIRC there's no standard bit for that in VGA
or PCI config registers, and I can't see any other  bit for it in the
docs. But I guess you could just turn off I/O space completely
via the PCI_COMMAND register. We shouldn't need it for anything beyond
i915_disable_vga() and that has the necessary vgaarb calls already.

-- 
Ville Syrjälä
Intel OTC
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