Re: Re: [Qemu-devel] [PATCH] Fix to gdb - wrong translation block invalidated when setting gdb breakpoints

[Andre Pech]

Hi Mulyadi,

The problem that you are running into here is that sys_uname has been
replaced by sys_newuname in kernel/sys.c. When I put a breakpoint in
this function, everything works correctly when I run uname in the
virtual machine.

I'm not sure I exactly understand your concern that breakpoints could
be missed. When you set the breakpoint, tb_invalidate_phys_page_range
is called, invalidating the translation block block for the address
where you are placing the breakpoint. At this point, the next time that
the address is hit, translate.c:gen_intermediate_code will have to be
called, and the breakpoint will be hit. Let me know if I've missed
something here.

Thanks,
Andre

On 1/1/06, Mulyadi Santosa <address@hidden> wrote:

Hello Andre...

> Not a problem. I only started using qemu a month ago, so it took me a
> while to get oriented in the code and understand what was going on. I
> must say that I've been really impressed with qemu so far.

There was an interesting case I had found recently. In Linux kernel for
i386 arch, you will see that sys_uname is placed to return kernel
version/name. Funny thing is, even if I use your patch (against qemu
0.7.1) and I put a breakpoint at sys_uname and issue "uname" at bash
prompt, the Qemu VM doesn't stop. Can you kindly check it?

NB: Please see target-i386/translate.c, there you will see lines like
these (around line 6306):
if (env->nb_breakpoints > 0) {
            for(j = 0; j < env->nb_breakpoints; j++) {
                if (env->breakpoints[j] == pc_ptr) {
                    gen_debug(dc, pc_ptr - dc->cs_base);
                    break;
                }
            }
        }
What I understand from this code is, VM is stop if breakpoint address
matches with pc_ptr, which tb->pc and AFAIK that is the start address
of the translation block. So in other word, in some cases Qemu might
still miss the breakpoint (does it explain the sys_uname case?) Please
CMIIW

regards

Mulyadi