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

[Blue Swirl <blauwirbel@gmail.com>]

On Sat, Feb 5, 2011 at 2:39 PM, Stefan Weil <weil@mail.berlios.de> wrote:
> 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 *).

Oh, that OS-Which-Must-Not-Be-Named.

> 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?

Does QEMU work in OS-Which-Must-Not-Be-Named when the patch is applied?

> Are there suggestions how it should be done?

The change, done properly, should not cause any problems for most
other hosts, where unsigned long size equals pointer type size.

> What about ram_addr_t? Should it be replaced by uintptr_t?

I'd use
typedef uintptr_t ram_addr_t;

> Should we use macros like QEMU_PTR2UINT(p), QEMU_UINT2PTR(u)?

Rather, inline functions if open coding is not clear.

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

Some comments:

The patch changes also signed longs to uintptr_t. That could introduce
regressions, so please use signed/unsigned as original.

For example in cpu_physical_memory_reset_dirty() you didn't change
length, but that should probably become uintptr_t too.

*/translate.c: exit_tb() helper uses tcg_target_long, so the cast
should use that instead.