Re: [Qemu-devel] Secondary VGA passthrough not working, hitting linux kernel's pci_fixup_video

[Bjorn Helgaas]

[+cc Jesse, David]

On Wed, Jan 15, 2014 at 12:19 PM, Sander Eikelenboom
<address@hidden> wrote:

>>> >>         I understood there is no bridge for a VGA device in your virtual 
>>> >> machine.
>>> >>         Your emulator for the bridge control register is odd.
>>>
>>> > That seems beside the point as there's no bridge here.
>>>
>>> >>         I guess your virtual machine ignore "PCI-to-PCI Bridge 
>>> >> Architecture
>>> >>         Specification".
>>>
>>> > Since there's no pci to pci bridge in this VM, why would this
>>> > specification apply?

My guess is that he is referring to sec 11.3.2 "Initialization
Algorithm" of the Bridge spec r1.2.  That section (and chapter 24 of
MindShare PCI System Architecture, 4th edition) apparently reiterate a
discovery algorithm that appears in the PCI spec.  I can't find that
algorithm in the 3.0 or 2.3 PCI specs, but I assume it was in an
earlier version.

> Yes i have done some more reading .. from what i could find from PCI specs 
> there is nothing in there
> that points to a specific boot vga card.
> The whole agp/pci/pcie bios selection is ACPI stuff .. but if all the cards 
> are of the same type ..
> there is no way to specify a specific card.

The algorithm from sec 11.3.2 *does* seem to give a way to identify a
specific boot VGA card, i.e., the first one you find when searching in
the specified order.  There are still issues, such as the fact that
11.3.2 says to start at PCI bus 0, while MindShare says to start with
the largest bus number.

Also, sec 12.1.2.1 of the Bridge spec says the PCI_COMMAND memory and
I/O enables do apply to VGA accesses, so that is potentially a way to
make two sibling VGA devices work, e.g., by toggling those bits to
control which device you want to talk to.  I also assume that when
identifying the "boot VGA device," one might ignore devices that the
BIOS left disabled.

>>> >> 00:03.0 VGA compatible controller [0300]: Cirrus Logic GD 5446 
>>> >> [1013:00b8] (prog-if 00 [VGA controller])
>>> >>         Subsystem: Red Hat, Inc Device [1af4:1100]
>>> >>         Physical Slot: 3
>>> >>         Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- 
>>> >> ParErr- Stepping- SERR- FastB2B- DisINTx-
>>> >>         Status: Cap- 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- 
>>> >> <TAbort- <MAbort- >SERR- <PERR- INTx-
>>> >>         Latency: 0
>>> >>         Region 0: Memory at f0000000 (32-bit, prefetchable) [size=32M]
>>> >>         Region 1: Memory at f30b0000 (32-bit, non-prefetchable) [size=4K]
>>> >>         Expansion ROM at f30a0000 [disabled] [size=64K]
>>> >>
>>> >> 00:05.0 VGA compatible controller [0300]: Advanced Micro Devices [AMD] 
>>> >> nee ATI Turks [Radeon HD 6570] [1002:6759] (prog-if 00 [VGA controller])
>>> >>         Subsystem: PC Partner Limited Device [174b:e193]
>>> >>         Physical Slot: 5
>>> >>         Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- 
>>> >> ParErr- Stepping- SERR- FastB2B- DisINTx+
>>> >>         Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- 
>>> >> <TAbort- <MAbort- >SERR- <PERR- INTx-
>>> >>         Latency: 0
>>> >>         Interrupt: pin A routed to IRQ 81
>>> >>         Region 0: Memory at e0000000 (64-bit, prefetchable) [size=256M]
>>> >>         Region 2: Memory at f3060000 (64-bit, non-prefetchable) 
>>> >> [size=128K]
>>> >>         Region 4: I/O ports at c100 [size=256]
>>> >>         Expansion ROM at f3080000 [disabled] [size=128K]

I am not an expert on this legacy stuff, but this looks like an
illegal configuration to me: both 00:03.0 and 00:05.0 claim to be VGA
controllers and both have I/O+ and Mem+, so I would think both would
respond to the legacy VGA addresses.