Re: [Qemu-devel] [PATCH] i386: fix breakpoints handling in icount mode

["Pavel Dovgaluk" <Pavel.Dovgaluk@ispras.ru>]

> From: Jan Kiszka [mailto:jan.kiszka@siemens.com]
> On 2014-10-22 13:38, Pavel Dovgalyuk wrote:
> > This patch fixes instructions counting when execution is stopped on
> > breakpoint (e.g. set from gdb). Without a patch extra instruction is translated
> > and icount is incremented by invalid value (which equals to number of
> > executed instructions + 1).
> >
> > Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
> > ---
> >  target-i386/translate.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/target-i386/translate.c b/target-i386/translate.c
> > index 1284173..193cf9f 100644
> > --- a/target-i386/translate.c
> > +++ b/target-i386/translate.c
> > @@ -8000,7 +8000,7 @@ static inline void gen_intermediate_code_internal(X86CPU *cpu,
> >                  if (bp->pc == pc_ptr &&
> >                      !((bp->flags & BP_CPU) && (tb->flags & HF_RF_MASK))) {
> >                      gen_debug(dc, pc_ptr - dc->cs_base);
> > -                    break;
> > +                    goto done_generating;
> >                  }
> >              }
> >          }
> > @@ -8049,6 +8049,7 @@ static inline void gen_intermediate_code_internal(X86CPU *cpu,
> >              break;
> >          }
> >      }
> > +done_generating:
> >      if (tb->cflags & CF_LAST_IO)
> >          gen_io_end();
> >      gen_tb_end(tb, num_insns);
> >
> >
> >
> 
> Didn't looked into why, just bisected that this patch breaks at least
> certain guest-originated break- or watchpoints in TCG mode. Can be
> triggered by booting a Linux kernel with kgdb self-tests enabled. The
> result is some false reporting of a host-originated debug stop to
> gdb_set_stop_cpu while gdbserver_state is NULL -> SEGV.

It seems that kernel sets some hardware breakpoints and QEMU tries to process 
them with GDB stub. Modifying gdb_set_stop_cpu should help:

diff --git a/gdbstub.c b/gdbstub.c
index e4a1a79..e8ef546 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1202,8 +1202,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
 
 void gdb_set_stop_cpu(CPUState *cpu)
 {
-    gdbserver_state->c_cpu = cpu;
-    gdbserver_state->g_cpu = cpu;
+    if (gdbserver_state) {
+        gdbserver_state->c_cpu = cpu;
+        gdbserver_state->g_cpu = cpu;
+    }
 }
 
 #ifndef CONFIG_USER_ONLY

Pavel Dovgalyuk