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 08:53:33PM +0000, Peter Maydell wrote:
> On 24 March 2013 19:17, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Sun, Mar 24, 2013 at 04:58:37PM +0100, Aurelien Jarno wrote:
> >> On Sun, Mar 24, 2013 at 11:32:30AM +0000, Peter Maydell wrote:
> >> > I've provided a property for enabling the old broken IRQ mapping;
> >> > this can be enabled with the command line option:
> >> >       -global versatile_pci.broken-irq-mapping=1
> >> >
> >> > (If anybody wants to suggest a better way of handling this please do.)
> >>
> >> Do you have a pointer to the corresponding kernel patch? Is it possible
> >> to get the kernel to detect if it should use the correct or the broken
> >> IRQ mapping?
> >
> > Alternatively, or additionally, how about detecting the correct or
> > the incorrect kernel and updating the mapping?
> > For example, maybe we could do this using the
> > IRQ value written into the device pci config register?
> 
> 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?)

Each pci device has a bit of memory where the OS stores
the IRQ#. I think it was invented as a simple way to pass
data from BIOS to the OS on a PC.
There's no special reason to store it on the device
but everyone does it.

> > If we can't find anything, maybe add our own register
> > so the same qemu config can support old and new kernels?
> 
> Then the new kernel wouldn't work on real hardware...
> 
> -- PMM

Not if our register is read-only in real hardware.

-- 
MST