On Fri, May 27, 2011 at 2:44 AM, Ian Campbell
On Fri, 2011-05-27 at 01:59 +0100, Kevin
On Thu, May 26, 2011 at 03:43:28PM +0100, Ian
Under Xen the VGA card ends up configured for
memory access only.
That's odd. How does the vga bios work if the Xen VGA device doesn't
handle inb/outb accesses?
I'm not entirely sure, but it does...
The Xen VGA device is the Cirrus GD 5446 provided by qemu. The I/O ports
are the VGA control registers at 0x3xx which are not covered by any PCI
BARS (FWIW this device has no I/O BARS, and two memory BARS).
It's not obvious whether the I/O space enable bit in the PCI cfg command
register is supposed to control the availability of non-PCI registers or
not. Neither the PCI spec nor the GD-5446 datasheet are particularly
clear on the matter.
I've just discovered that the ancient pentium box I use as a home
firewall has a GD 5446 in it (useful to know!), it doesn't have any I/O
BARS but it does have the I/O bit set in the PCI command register.
It's not clear who was responsible for setting that bit, in general in
the absence of an I/O BAR the BIOS wouldn't know to do so. I expect that
either the VGA BIOS is expected to enable it if the hardware it drives
requires it or that BIOSen special case class=VGA devices and always
enable I/O for one of them.
It looks like coreboot always forces this bit on for the VGA device
which it determines to be the primary, which is good enough for me --
I'll make a patch to the Xen pci setup code to implement that instead of
this change to SeaBIOS.
There are some exceptions around VGA and PCI. The primary display is
usually the first VGA device found, by checking device class code.
Enabling the IO bit on a VGA device causes it to claim the legacy
cycles. It doesn't matter if the device has an IO bar.
If the VGA device also has the device's palette snoop bit set, it
claims reads and snarfs writes allowing a downstream device to also
get the writes. Note that there is also the VGA enable to be set on
bridges above the device and VGA palette snoop for bridges that a VGA
device is behind below the primary device bus.