Re: [Qemu-devel] [PATCH 00/10] Fix versatile_pci (and break versatilepb linux guests!)

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Mar 24, 2013 at 09:16:28PM +0000, Arnd Bergmann wrote:
> On Sunday 24 March 2013, Peter Maydell wrote:
> > Yeah, ideally being able to detect the buggy kernel would be good;
> > I can't see anything at the controller level that would do though,
> > and I don't really know enough about PCI to know about generic
> > PCI stuff that would work. (Why would the OS need to tell the
> > device anything about its IRQ if it's hardwired?)
> 
> I think it actually does on versatile and other platforms on which
> the kernel probes the PCI bus itself, rather than relying on firmware
> to have resources assigned in advance.
> 
> IIRC, the PCI_INTERRUPT_LINE pci config space byte (0x3c) is purely
> informational and used as a way to communicate the interrupt number
> from the bus scan code (assumed to be a PC BIOS in the PCI spec,
> but drivers/pci/setup-irq.c in case of versatile+linux) to a device
> driver.
> 
> So the kernel should actually write the proper interrupt number in
> there. In future kernels, this may not necessarily be the hardware
> number, but today it is.

For future kernels, let's build in some hook that let
qemu detect a non broken guest. How about writing
some magic value into revision ID or some other
readonly field?

> Can you try out what the kernel writes into
> that register in qemu, with and without my patches?
> 
> I would expect the numbers to be (64+27) to (64+30), since we
> linearize the interrupt numbers so that VIC gets 32 through
> 63 and SIC gets 64 through 95.
> 
> 	Arnd