Guidance on emulating "sparse" address spaces

[Jason Thorpe <thorpej@me.com>]

As a "learn the internals of Qemu a little better" exercise, I am planning to write models for some older Alpha systems, initially for one based on the LCA45.  One of the quirks of these old systems, though, is lack of byte/word load/store.  So, to support 8- and 16-bit accesses to I/O devices, the PCI interfaces on these systems implement "sparse" I/O and memory spaces (some also implement a "dense" space that can be used for e.g. frame buffers; more on that another time).

The way the sparse spaces work is that the address space is exploded out, and the CPU-visible address used to perform the access is computed using the desired bus address along with a field to specify the byte-enables.

Using the 21066's IOC as an example, PCI I/O addresses 0000.0000 - 00ff.ffff are mapped to 1.c000.0000 - 0x1.dfff.fff.  The offset into the I/O space is shifted left 5 bits, and the byte-enables code is shifted left 3 bits, and both are added to the base address of PCI I/O space in the system memory map (1.c000.0000) resulting in the system physical address to use in a 32-bit load/store.  Software then does e.g. a 32-bit read to that location, and extracts the value out the relevant field.

Further complicating things ... it's possible for the bus region that's mapped into the system address space to not begin at 0.  As a hypothetical example you might have a PCI sparse memory space that maps PCI memory addresses 1000.0000 - 1fff.ffff.  The 2117x chipsets used on EV5/EV56 is a concrete example of a PCI interface that implements multiple windows for each space type.

I'm trying to wrap my head around how to achieve this in Qemu.  I don't see an obvious way from my initial study of how the PCI code and memory regions work.  Some guidance would be appreciated!

Thx.

-- thorpej