Re: [Qemu-devel] Qemu / KQemu on 64-bit (x86_64) host systems

[andrzej zaborowski]

Hi,

On 16/04/07, Werner Dittmann <address@hidden> wrote:
> During several tests with Qemu / Kqemu it seems that Qemu
> has problems with x86_64 host systems. My system is an
> AMD 64 X2 (Dual Core), running openSUSE 10.2, 2GB memory.
>
> Various versions of Qemu/Kqemu available and under test:
> 0.8.2, 0.9.0, and CVS. Kqemu 1.3.0pre9, 1.3.0pre11
>
> When building Qemu I use the following configure setup,
> using a gcc 3.4:
> ./configure --prefix=/usr/local/ \
>  --cc=/opt/gcc34/bin/gcc-3.4 --host-cc=/opt/gcc34/bin/gcc-3.4 \
>  --enable-alsa  --enable-adlib \
>  --target-list="i386-softmmu x86_64-softmmu"
>
> Kqemu built with standard (system) gcc.
>
> I always use qemu-system-x86_64 to start Qemu.
>
> Here the problems:
>
> Installing a 32bit Linux system (Debian, Kernel 2.6.18):
> - works with pure Qemu (-no-kqemu)
> - fails with Kqemu support enabled. The failure is a loop
>   before or during the kernel hands over control to INIT

Does your host happen to be dual-core? If so, please try adding
"notsc" to the guest kernel commandline and report if it makes a
difference.

> I used gdb to get some more information about the problems
> using the following command:
>  gdb qemu-system-x86_64
>
> using a .gdbinit that sets the args, etc.
>
> When the kernel goes into the loop I interrupt with ^C
> several times, most of the time it was in code_gen_buffer,
> here in the function "compute_c_subl".
>
> Because I'm _not_ sure this is the correct way to debug Qemu
> I cannot say if this is normal or not. At least the function
> always returns  1 (it seems that it is called over and over
> again with). The last relevant statement in this function is:
>
> cmp  %eax,0x90(%r14)
> seta %al
>
> where the conetent of %eax is zero, the content of the memory
> is 0x00000000ffffeb3e. The return says: the memory content is
> bigger than 0x0 (which is true for 64bit, but also true for
> 32bit unsigned, "compute_c_subl" compares two unsigned 32bit
> integers). As said, take these findings with a grain of
> salt.
>
> My general thought about the problem: running 32bit code
> on a 64bit host with similar architecture as this is the case
> of x86 / x86_64 could easily result in problems with signedness,
> sign bit extension, different pointer/word/interger sizes...
>
> BTW: is there a Howto or other information how to debug
> Qemu when the loaded kernel loops or crashes? That would be
> great and would make it easier to step in here and provide some
> help (or is this a somewhat good kept secret :-) ? ).

Use qemu's gdb server, it's documented.

> The next problems are fairly old, they are also reported in the
> Qemu user's wiki - but without an answer o solution.
>
> Installing a 64bit Linux system (openSuse 10.1, 10.2):
> - fails with Qemu (-no-kqemu), loops when Grub shall install
>   the bootloader.
> - fails with Kqemu enabled, crashes at various addresses and
>   prints register contents.
>
> Any hints what this could be? Solutions?
>
> Regards,
> Werner

Regards,
Andrzej