Re: [Qemu-devel] AArch64: QEMU fails in swapcontext

[Mian M. Hamayun]

On 18/04/2014 16:44, Richard Henderson wrote:

On 04/18/2014 07:00 AM, Mian M. Hamayun wrote:

Hello Peter & All,

I am trying to figure out a problem in qemu on aarch64 (with kvm enabled). I
have found this problem in many different versions of qemu
(v2.0.0-rc3/rc2/rc1/rc0, master 2d03b49), and I believe that either I am
missing something common in all of these versions or its a genuine bug in qemu
on aarch64.

The problem is triggered by virtqueue_notify() function (in virtio_ring.c) from
the guest kernel and fails in the qemu_coroutine_new() while trying to do the
swapcontext(&old_uc, &uc) (see coroutine-ucontext.c:164). The
sigsetjmp(old_env, 0) just before the swapcontext() call seems to work fine, as
it returns 0, and then we invoke the swapcontext().

The host kernel reports:
"qemu-system-aar[596]: bad frame in sys_rt_sigreturn: pc=004462e0
sp=7f8020f000" and kills the qemu process due to segmentation fault. The
pc=004462e0 is for the coroutine_trampoline() but we don't actually reach it,
when this particular crash happens.

Just to give you an idea of the code I am talking about:

$~/qemu[master]$ git blame -L 159,166 coroutine-ucontext.c
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 159)     makecontext(&uc,
(void (*)(void))coroutine_trampoline,
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 160)                 2,
arg.i[0], arg.i[1]);
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 161)
6ab7e546 (Peter Maydell 2013-02-20 15:21:09 +0000 162)     /* swapcontext() in,
siglongjmp() back out */
6ab7e546 (Peter Maydell 2013-02-20 15:21:09 +0000 163)     if
(!sigsetjmp(old_env, 0)) {
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 164)        
swapcontext(&old_uc, &uc);
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 165)     }
00dccaf1 (Kevin Wolf    2011-01-17 16:08:14 +0000 166)     return &co->base;

My qemu configure/run commands are:

./configure --target-list=aarch64-softmmu       \
    --cross-prefix=aarch64-linux-gnu-           \
    --enable-fdt *--enable-kvm* --disable-werror  \
    --audio-drv-list="" --static

./qemu-system-aarch64 \
    *-enable-kvm* -nographic -kernel Image    \
    -drive if=none,file=disk_oe64.img,id=fs \
    -device virtio-blk-device,drive=fs      \
    -m 1024 -M virt -cpu host               \
    -append "earlyprintk console=ttyAMA0 mem=1024M rootwait root=/dev/vda rw
init=/bin/sh"

Any ideas/comments on how to resolve this issue?

Note that a patch has just gone into glibc to rewrite setcontext et al for
aarch64.  I'd try using git glibc before looking too much deeper.


r~

It seems right to me as well, as the setcontext related bug (for aarch64) has recently been posted/fixed in glibc:
https://sourceware.org/bugzilla/show_bug.cgi?id=16629

I have also tested the example posted at: https://sourceware.org/bugzilla/attachment.cgi?id=7435
and I get the following output on x86_64:

start f2
start f1
finish f2
finish f1
{ss_sp: (nil), ss_flags: 2, ss_size: 0}
{ss_sp: (nil), ss_flags: 2, ss_size: 0}

and this one on AArch64:

start f2
start f1
finish f2
finish f1
{ss_sp: (nil), ss_flags: 2, ss_size: 0}
{ss_sp: 0x7ff7e723f0, ss_flags: 0, ss_size: 8192}

I will first look into updating my glibc for AArch64 before coming back to qemu.

Thanks for your support,
Hamayun