On Wed, 10 Aug 2016 17:19:48 +0200 Paolo Bonzini <pbonz...@redhat.com> wrote:
CCing original author, here is his varinat how to fix it https://www.mail-archive.com/qemu-devel@nongnu.org/msg391790.html > On 10/08/2016 03:05, Brent W. Baccala wrote: > > Hi - > > > > This is a bug report that includes a patch. My understanding of your > > submission guidelines in that it should go to qemu-devel and not to the bug > > tracker. Sorry if I didn't understand your procedures. > > > > I'm running qemu-system-i386 built from the master branch from git:// > > git.qemu-project.org/qemu.git. > > > > The problem shows up with a Debian/Hurd guest using remote GDB on the Mach > > kernel. Setting a breakpoint in the kernel with GDB and then hitting the > > breakpoint triggers a SEGV in qemu, i.e: > > > > baccala@baccala:~/src/qemu$ gdb i386-softmmu/qemu-system-i386 > > (gdb) run -enable-kvm -m 512 -nographic -drive cache=writeback,format=raw, > > file=/home/baccala/hurd/debian-hurd.img -net nic -net > > user,hostfwd=tcp::3333-:22 -s > > > > After Hurd boots, in a different window, I attach GDB and set a kernel > > breakpoint: > > > > baccala@baccala:~/hurd$ gdb gnumach-17-486-dbg > > (gdb) target remote localhost:1234 > > Remote debugging using localhost:1234 > > machine_idle (cpu=0) at ../i386/i386at/model_dep.c:207 > > 207 ../i386/i386at/model_dep.c: No such file or directory. > > (gdb) where > > #0 machine_idle (cpu=0) at ../i386/i386at/model_dep.c:207 > > #1 0x810293fa in idle_thread_continue () at ../kern/sched_prim.c:1674 > > #2 0x00000000 in ?? () > > (gdb) break elf_db_search_symbol > > Breakpoint 1 at 0x8100d080: file ../ddb/db_elf.c, line 159. > > (gdb) cont > > Continuing. > > > > Now, I trigger the kernel breakpoint (by hitting Cntl-Alt-d to enter the > > Mach debugger), and qemu segfaults. Here's what GDB on qemu itself shows: > > > > Thread 1 "qemu-system-i38" received signal SIGSEGV, Segmentation fault. > > 0x0000555555b9d0c3 in qht_reset_size (ht=0x55555617f698 <tcg_ctx+216>, > > n_elems=32768) at util/qht.c:422 > > 422 if (n_buckets != map->n_buckets) { > > (gdb) where > > #0 0x0000555555b9d0c3 in qht_reset_size (ht=0x55555617f698 <tcg_ctx+216>, > > n_elems=32768) at util/qht.c:422 > > #1 0x000055555573f9d1 in tb_flush (cpu=0x0) at /home/baccala/src/qemu/ > > translate-all.c:855 > > #2 0x000055555577ac96 in gdb_vm_state_change (opaque=0x0, running=0, > > state=RUN_STATE_DEBUG) > > at /home/baccala/src/qemu/gdbstub.c:1276 > > #3 0x000055555589d167 in vm_state_notify (running=0, > > state=RUN_STATE_DEBUG) at vl.c:1585 > > #4 0x000055555576bd9c in do_vm_stop (state=RUN_STATE_DEBUG) at > > /home/baccala/src/qemu/cpus.c:743 > > #5 0x000055555576d550 in vm_stop (state=RUN_STATE_DEBUG) at > > /home/baccala/src/qemu/cpus.c:1476 > > #6 0x000055555589d7c4 in main_loop_should_exit () at vl.c:1856 > > #7 0x000055555589d933 in main_loop () at vl.c:1912 > > #8 0x00005555558a4ee7 in main (argc=12, argv=0x7fffffffddc8, > > envp=0x7fffffffde30) at vl.c:4603 > > (gdb) > > > > "map" is NULL at this point. I'm not sure what should happen when > > ght_reset_size() gets called on a hash tree with a NULL map. I applied the > > following patch and everything seems to work. I can now hit kernel > > breakpoints and step/next through the Mach kernel code without segfaults > > from qemu. I don't know qemu, so it's just an educated guess. > > > > agape > > brent > > > > Signed-off-by: Brent Baccala <cos...@freesoft.org> > > --- > > util/qht.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/util/qht.c b/util/qht.c > > index 16a8d79..9af21e3 100644 > > --- a/util/qht.c > > +++ b/util/qht.c > > @@ -419,6 +419,14 @@ bool qht_reset_size(struct qht *ht, size_t n_elems) > > > > qemu_mutex_lock(&ht->lock); > > map = ht->map; > > + > > + if (!map) { > > + new = qht_map_create(n_buckets); > > + atomic_rcu_set(&ht->map, new); > > + qemu_mutex_unlock(&ht->lock); > > + return true; > > + } > > + > > if (n_buckets != map->n_buckets) { > > new = qht_map_create(n_buckets); > > resize = true; > > > > The patch makes sense, but I think we don't need to call qht_reset_size > at all. > > tb_flush should not do anything if using KVM. There are several ways to > do this: > > - put the tb_flush call under "if (tcg_enabled())" > > - add an "if (!tcg_enabled()) return;" in tb_flush > > - add an "if (!tcg_ctx.tb_ctx.nb_tbs) return;" in tb_flush > > Thanks, > > Paolo >
