[Qemu-devel] Porting QEMU to new hosts with unusual ABI (sizeof(long) != sizeof(void *))

[Stefan Weil <weil@mail.berlios.de>]

Currently, most QEMU code assumes that pointers and long integers have
the same size, typically 32 bit on 32 bit hosts, 64 bit on 64 bit hosts.

While this assumption works on QEMU's major hosts, it is not generally true.

There exist 64 bit host OS which use an ABI with 32 bit long integers,
maybe to be more compatible with an older 32 bit OS version, so here is
sizeof(long) < sizeof(void *).

Other ABIs might use "near" pointers which may reduce code size and improve
code speed. This results in sizeof(long) > sizeof(void *).

Both cases will break current QEMU, because lots of code lines use
type casts from pointer to long or vice versa like these two examples:

start = (long)mmap((void *)host_start, host_len ...
code_gen_ptr = (void *)(((unsigned long)code_gen_ptr + ...))

Both variants (unsigned long) and (long) can be found (relation 3:2).

Changing the existing limitation of QEMU's code simply needs replacing
all those type casts, variable declarations and printf format specifiers
by portable code.

The standard integral type which matches the size of a pointer is defined
in stdint.h (which also defines int8_t, ...). It is intptr_t (signed 
version)
or uintptr_t (unsigned version). There is no need to use both.

=> Replace (unsigned long), (long) type casts of pointers by (uintptr_t).

All variables (auto, struct members, parameters) which hold such values
must be fixed, too. In the following examples, ptr_var is of that kind.

=> Replace unsigned long ptr_var, long ptr_var by uintptr_t ptr_var.

Finally, the fixed variables are used in printf-like statements,
so here the format specifier needs a change. inttypes.h defines
PRIxPTR which should be used.

=> Replace "%lx" by "%" PRIxPTR to print the integer value ptr_var.

A single patch which includes all these changes touches 39 files.
Splitting it into smaller patches is not trivial because of cross
dependencies. Because of its size, it will raise merge conflicts
when it is not applied soon.

Would these kind of changes be interesting for QEMU?
Are there suggestions how it should be done?
What about ram_addr_t? Should it be replaced by uintptr_t?
Should we use macros like QEMU_PTR2UINT(p), QEMU_UINT2PTR(u)?

My current version of the patch is available from
http://qemu.weilnetz.de/0001-Fix-conversions-from-pointer-to-integral-type-and-vi.patch.

Kind regards,
Stefan Weil