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Re: [Qemu-devel] how do we determine correct guest PC for segfaults in a

From: Philippe Mathieu-Daudé
Subject: Re: [Qemu-devel] how do we determine correct guest PC for segfaults in atomic helpers for linux-user mode?
Date: Mon, 13 Nov 2017 19:53:27 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

Hi Peter,

On 11/13/2017 04:59 PM, Peter Maydell wrote:
> I've been investigating a bug (a javac crash). I'm not sure if it's
> the root cause, but I can't figure out how, if we get a guest SEGV in
> an atomic helper we report the right faulting PC to the guest.
> Specifically, if you get a SEGV here:
> #0  0x000000006003c22b in helper_atomic_cmpxchgl_le (env=0x63caf680,
>     addr=275041819628, cmpv=0, newv=1)
>     at 
> /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/atomic_template.h:65
> #1  0x0000000061002f61 in static_code_gen_buffer ()
> #2  0x0000000060035d6b in cpu_tb_exec (cpu=0x63ca73e0,
>     itb=0x6119d000 <static_code_gen_buffer+9080960>)
>     at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:167
> #3  0x0000000060036945 in cpu_loop_exec_tb (cpu=0x63ca73e0,
>     tb=0x6119d000 <static_code_gen_buffer+9080960>, last_tb=0x7f01b213dbd8,
>     tb_exit=0x7f01b213dbd0)
>     at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:611
> #4  0x0000000060036bc2 in cpu_exec (cpu=0x63ca73e0)
>     at /home/petmay01/linaro/qemu-from-laptop/qemu/accel/tcg/cpu-exec.c:723
> #5  0x000000006003da13 in cpu_loop (env=0x63caf680)
>     at /home/petmay01/linaro/qemu-from-laptop/qemu/linux-user/main.c:809
> #6  0x000000006004c627 in clone_func (arg=0x7ffe028f0a10)
>     at /home/petmay01/linaro/qemu-from-laptop/qemu/linux-user/syscall.c:6241
> #7  0x00000000602fcc25 in start_thread (arg=0x7f01b213e700)
>     at pthread_create.c:333
> #8  0x00000000603949a9 in clone ()
> then the code in handle_cpu_signal() is passed a pc of 0x6003c22b
> (the location in the helper function that does the memory access).
> This is outside generated code, so the call to cpu_restore_state()
> in handle_cpu_signal() will do nothing. However as far as I can tell,
> there isn't any syncing of the PC etc state to the CPU before calling
> this helper (at least, env->pc is completely wrong for the insn that
> I think is causing this helper call).

I'm not sure this is related, but last week I hit a similar problem when
my laptop ran Out Of Memory using the xlnx-zcu102 machine; but I wasn't
getting a SEGV but various SIGBUS in different places, and my backtraces
don't show atomic_template.h but softmmu_template.h.

I found it is easier to understand a such OOM using the -mem-prealloc
option, I now get a more OOM-related error:

Thread 12 "qemu-system-aar" received signal SIGBUS, Bus error.
0x0000555555de1bd4 in do_touch_pages (arg=0x555556cc0210) at
331            *(volatile char *)addr = *addr;



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