[Qemu-devel] Memory API: handling unassigned physical memory

[Mark Cave-Ayland]

Hi all,

I've been having a look at handling SBUS probes within qemu-system-sparc 
when I came across a very simple crash bug with git master trying to 
access unassigned physical addresses:

(qemu) info mtree
memory
0000000000000000-7ffffffffffffffe (prio 0, RW): system
  0000000000000000-0000000007ffffff (prio 0, RW): sun4m.ram
  0000000008000000-000000000fffffff (prio 0, RW): empty-slot
  0000000010000000-0000000010003fff (prio 0, RW): iommu
  0000000010004000-000000001fffefff (prio 0, RW): empty-slot
  0000000050200000-000000005020000f (prio 0, RW): tcx.dac
  ...
  ...

AFAICT the devices are mapped to physical addresses (uandsing 
sysbus_mmio_map) so the xp monitor command should be able to at least 
read these regions:

(qemu) xp 0x50200000
0000000050200000: 0x00000000
(qemu) xp 0x501ffff0
Segmentation fault

There is definitely a QEMU bug here in that an incorrect physical memory 
access shouldn't segfault QEMU. However some off-list discussions with 
Blue suggested that it may be possible to defer this behaviour to the 
memory API (rather than in exec.c) by allowing boards to register a 
simple device to handle unassigned memory accesses, e.g. like the 
empty_slot device used to trap certain memory accesses within SPARC.

This would enable us to easily solve the problem for SPARC since we 
could create a parent memory region for the entire physical address 
space that would simply update the status register and raise the 
required IRQ. Is this the best way to solve the problem or is there 
something else that I've missed?


Many thanks,

Mark.