Re: [Qemu-devel] RFC: fix run of 32 bits Linux executables on 64 bits targets

[J. Mayer]

On Thu, 2007-10-11 at 22:26 +0300, Blue Swirl wrote:
> On 10/10/07, Fabrice Bellard <address@hidden> wrote:
> > Thiemo Seufer wrote:
> > > Fabrice Bellard wrote:
> > >> J. Mayer wrote:
> > >>> Following the patches done for elfload32, it appeared to me that there
> > >>> were still problems that would prevent 32 bits executables to run on 64
> > >>> bits target in linux user mode emulation.
> > >>> [...]
> > >> Are you sure it is a good idea to try to add 32 bit executable support 
> > >> to a
> > >> 64 bit target ? In the end you will need to write a 64 bit to 32 bit 
> > >> linux
> > >> syscall converter which would mean duplicating all the linux-user code of
> > >> the corresponding 32 bit target (think of ioctls with strutures, signals
> > >> frames, etc...).
> > >
> > > I would think this feature will be limited to platforms which can handle
> > > 32bit and 64bit binaries with a single personality.
> >
> > I am not sure it is a common case !
> >
> > However, I suggest to emulate a 32 bit user linux system with a 64 bit
> > guest CPU running in 32 bit compatibily mode. It would be useful to test
> > 64 bit CPUs in 32 bit compatibility mode. The only required modification
> > in linux user is to rename target_ulong so that it can have a different
> > size of the CPU word default size.
> 
> I made a patch to rename target_ulong/long to abi_ulong/long and also
> add a new emulator target that uses the 32 bit ABI with 64 bit CPU.
> 
> Some Sparc32 binaries run, others don't, possibly indicating bugs in
> the Sparc64 emulation!
> 
> The patch is quite large because of the renaming, but this shouldn't
> have effect to any other target. Any comments?

Great !
The patch seems safe, at first look, then I noticed a few things that
are not correct or may be improved:
* In linux-user/main.c: PowerPC DCR access should keep using
target_ulong. This is a hardware bus, not an ABI dependent stuff. If a
32 bits cast is needed, it would be done in the micro-ops that handle
the DCR bus accesses.
* in linux-user/qemu.h: why is there still a OVERRIDE_ELF_CLASS
variable, when checking TARGET_ABI32 should be sufficient ? It seems to
me that having 2 defines which are, in fact, synonymous may be a source
of confusion.
* in configure: you also added a sparc64-softmmu target, which seems not
related with this particular patch.
* in configure: why add a specific TARGET_ABI32_DIR variable for that
case ? It seems to me that a TARGET_ABI_DIR variable could be useful for
all targets. Let me give an example: I want to add a ppcemb-linux-user
target, emulating a PowerPC 32 with 64 bits registers and SIMD
extensions but I don't want to duplicate the linux-user/ppc
subdirectory. Having a TARGET_ABI_DIR available for all targets would
solve my problem. In fact, even ppc and ppc64 could be merged... As you
need this feature in your case, I think it would be a good idea to add
it for all targets. And then, the kludge in Makefile.target could be
replaced by:
-CPPFLAGS+=-I$(SRC_PATH)/linux-user -I
$(SRC_PATH)/linux-user/$(TARGET_ARCH)
+CPPFLAGS+=-I$(SRC_PATH)/linux-user -I
$(SRC_PATH)/linux-user/$(TARGET_ABI_DIR)
which is simpler and easier to understand, imho.

-- 
J. Mayer <address@hidden>
Never organized