[Qemu-devel] Re: qemu crashes and freezes on x86_64/amd64 host

[Karl Magdsick <kmagnum@gmail.com>]

But my point is that if it's compiled for amd64, then the kernel will
see in the ELF header that it is 64-bit code and will not switch to
32-bit compat mode for that process's threads.  Unless someone has
re-written the dynamic code generator to generate amd64 code, then the
dynamic code generator is generating 32-bit code that's then being
executed in non-32-bit-compat mode, due to the rest of the code being
64-bit code.

It's like linking 64-bit object code and 32-bit object code into the
same executible.  It may very well be the case that the rest of the
code isn't completely 64-bit clean.  However, I believe there is a
fundamental problem in calling 32-bit code from 64-bit code without
switching into 32-bit-compat mode (which I would guess is a privledged
instruction).

Under Linux, if you are willing to use raw clone system calls instead
of libpthread, you may be able to create a 32-bit compat thread whose
entire address space is a subset of your 64-bit address space and you
may be able to keep that 32-bit thread inside the dynamically
generated code (maybe having the 32-bit thread set a pointer for its
next jump and then wait on a mutex or semaphore while the 64-bit
threads create the 32-bit code corresponding to that pointer). 
However, it would be much easier to just modify the x86 dynamic code
generator to emit amd64 code.


-Karl


On Sat, 09 Oct 2004 18:19:37 +0200, Andi Kleen <ak@muc.de> wrote:
> Karl Magdsick <kmagnum@gmail.com> writes:
> 
> > What is your compiler target when creating the qemu executible?
> >
> > My understanding is that almost all of the instructions in 64-bit mode
> > are reverse-compatible with 32-bit mode, but a few have changed
> > slightly.  If your qemu executible is compiled for 64-bit mode (and is
> > therefore being run in 64-bit usermode), but the jit is generating
> > 32-bit code, this could be problematic for a small number of
> > instructions.  I have no direct knowledge of the differences between
> > the instruction encodings for 32-bit and 64-bit modes, just hearsay.
> 
> That's basically correct, but x86-64 has a "compat mode" that executes
> old 32bit programs without changes. When qemu has been compiled
> as 32bit program it will run in compat mode.
> 
> The main reason things crash there is that it gives these programs
> by default 4GB of address space. An i386 kernel defaults to 3GB.
> You can force the 3GB address space with linux32 --3gb ...
> 
> I tested an older 32bit qemu version and it worked for me on an 64bit
> kernel. Wasn't able to compile a new one so far because someone
> added a bogus dependency on arts.
> 
> -Andi
> 
>