[Qemu-devel] [PATCH] gdb: Fix gdb error

[Qemu-devel] [PATCH] gdb: Fix gdb error

[Aneesh Kumar K.V]

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>

Don't update the global register count if not requested.
Without this patch a remote gdb session gives

(gdb) target remote localhost:1234
Remote debugging using localhost:1234
Remote 'g' packet reply is too long:
0000000028000084c000000000ccba50c000000000c ...
....
...
(gdb)

This is a regression introduce by a0e372f0c49ac01faeaeb73a6e8f50e8ac615f34

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
---
 gdbstub.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/gdbstub.c b/gdbstub.c
index 1af25a6..4b58a1e 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -598,6 +598,12 @@ void gdb_register_coprocessor(CPUState *cpu,
 {
     GDBRegisterState *s;
     GDBRegisterState **p;
+    static int last_reg;
+    CPUClass *cc = CPU_GET_CLASS(cpu);
+
+    if (!last_reg) {
+        last_reg = cc->gdb_num_core_regs;
+    }
 
     p = &cpu->gdb_regs;
     while (*p) {
@@ -608,19 +614,21 @@ void gdb_register_coprocessor(CPUState *cpu,
     }
 
     s = g_new0(GDBRegisterState, 1);
-    s->base_reg = cpu->gdb_num_regs;
+    s->base_reg = last_reg;
     s->num_regs = num_regs;
     s->get_reg = get_reg;
     s->set_reg = set_reg;
     s->xml = xml;
 
     /* Add to end of list.  */
-    cpu->gdb_num_regs += num_regs;
+    last_reg += num_regs;
     *p = s;
     if (g_pos) {
         if (g_pos != s->base_reg) {
             fprintf(stderr, "Error: Bad gdb register numbering for '%s'\n"
                     "Expected %d got %d\n", xml, g_pos, s->base_reg);
+        } else {
+            cpu->gdb_num_regs = last_reg;
         }
     }
 }
-- 
1.8.1.2

Re: [Qemu-devel] [PATCH] gdb: Fix gdb error

[Andreas Färber]

Hi Aneesh,

Am 11.08.2013 20:14, schrieb Aneesh Kumar K.V:
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> 
> Don't update the global register count if not requested.
> Without this patch a remote gdb session gives
> 
> (gdb) target remote localhost:1234
> Remote debugging using localhost:1234
> Remote 'g' packet reply is too long:
> 0000000028000084c000000000ccba50c000000000c ...
> ....
> ...
> (gdb)
> 
> This is a regression introduce by a0e372f0c49ac01faeaeb73a6e8f50e8ac615f34
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

Thanks for tracking this down. I'm willing to include a variation in
today's pull to fix 1.6.0-rc3. However, did you find an explanation
*why* it needs to be like this? I understand it is a revert to using the
static variable, updated to using the CPUClass field rather than the
previous preprocessor constant.

> ---
>  gdbstub.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/gdbstub.c b/gdbstub.c
> index 1af25a6..4b58a1e 100644
> --- a/gdbstub.c
> +++ b/gdbstub.c
> @@ -598,6 +598,12 @@ void gdb_register_coprocessor(CPUState *cpu,
>  {
>      GDBRegisterState *s;
>      GDBRegisterState **p;
> +    static int last_reg;
> +    CPUClass *cc = CPU_GET_CLASS(cpu);
> +
> +    if (!last_reg) {
> +        last_reg = cc->gdb_num_core_regs;
> +    }
>  
>      p = &cpu->gdb_regs;
>      while (*p) {
> @@ -608,19 +614,21 @@ void gdb_register_coprocessor(CPUState *cpu,
>      }
>  
>      s = g_new0(GDBRegisterState, 1);
> -    s->base_reg = cpu->gdb_num_regs;
> +    s->base_reg = last_reg;
>      s->num_regs = num_regs;
>      s->get_reg = get_reg;
>      s->set_reg = set_reg;
>      s->xml = xml;
>  
>      /* Add to end of list.  */
> -    cpu->gdb_num_regs += num_regs;
> +    last_reg += num_regs;
>      *p = s;
>      if (g_pos) {
>          if (g_pos != s->base_reg) {
>              fprintf(stderr, "Error: Bad gdb register numbering for '%s'\n"
>                      "Expected %d got %d\n", xml, g_pos, s->base_reg);

> +        } else {
> +            cpu->gdb_num_regs = last_reg;

This bit looks wrong to me - it is updating the per-CPU count with the
global value. Could you retest without this please?

Regards,
Andreas

>          }
>      }
>  }
> 


-- 
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg

Re: [Qemu-devel] [PATCH] gdb: Fix gdb error

[Aneesh Kumar K.V]

Andreas Färber <afaerber@suse.de> writes:

> Hi Aneesh,
>
> Am 11.08.2013 20:14, schrieb Aneesh Kumar K.V:
>> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>> 
>> Don't update the global register count if not requested.
>> Without this patch a remote gdb session gives
>> 
>> (gdb) target remote localhost:1234
>> Remote debugging using localhost:1234
>> Remote 'g' packet reply is too long:
>> 0000000028000084c000000000ccba50c000000000c ...
>> ....
>> ...
>> (gdb)
>> 
>> This is a regression introduce by a0e372f0c49ac01faeaeb73a6e8f50e8ac615f34
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>
> Thanks for tracking this down. I'm willing to include a variation in
> today's pull to fix 1.6.0-rc3. However, did you find an explanation
> *why* it needs to be like this?

IIUC our reply packet for 'g' contain more data becaue we ended up with
larger cpu->gdb_num_regs. This only happens for archs that do a
gdb_register_coprocessor with gpos == 0. The older code didn't update
num_g_regs in that case. Not sure why we do like that

> I understand it is a revert to using the
> static variable, updated to using the CPUClass field rather than the
> previous preprocessor constant.
>

I don't really like the patch. But I also don't know enough to fix this
without using the static variable.  If you want me to try another
version please send it across. I can easily reproduce this on PowerPC.

>> ---
>>  gdbstub.c | 12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>> 
>> diff --git a/gdbstub.c b/gdbstub.c
>> index 1af25a6..4b58a1e 100644
>> --- a/gdbstub.c
>> +++ b/gdbstub.c
>> @@ -598,6 +598,12 @@ void gdb_register_coprocessor(CPUState *cpu,
>>  {
>>      GDBRegisterState *s;
>>      GDBRegisterState **p;
>> +    static int last_reg;
>> +    CPUClass *cc = CPU_GET_CLASS(cpu);
>> +
>> +    if (!last_reg) {
>> +        last_reg = cc->gdb_num_core_regs;
>> +    }
>>  
>>      p = &cpu->gdb_regs;
>>      while (*p) {
>> @@ -608,19 +614,21 @@ void gdb_register_coprocessor(CPUState *cpu,
>>      }
>>  
>>      s = g_new0(GDBRegisterState, 1);
>> -    s->base_reg = cpu->gdb_num_regs;
>> +    s->base_reg = last_reg;
>>      s->num_regs = num_regs;
>>      s->get_reg = get_reg;
>>      s->set_reg = set_reg;
>>      s->xml = xml;
>>  
>>      /* Add to end of list.  */
>> -    cpu->gdb_num_regs += num_regs;
>> +    last_reg += num_regs;
>>      *p = s;
>>      if (g_pos) {
>>          if (g_pos != s->base_reg) {
>>              fprintf(stderr, "Error: Bad gdb register numbering for '%s'\n"
>>                      "Expected %d got %d\n", xml, g_pos, s->base_reg);
>
>> +        } else {
>> +            cpu->gdb_num_regs = last_reg;
>
> This bit looks wrong to me - it is updating the per-CPU count with the
> global value. Could you retest without this please?
>

We loop with cpu->gdb_num_regs as below in gdb_handle_packet.


-        for (addr = 0; addr < num_g_regs && len > 0; addr++) {
+        for (addr = 0; addr < s->g_cpu->gdb_num_regs && len > 0; addr++) {

We updated num_g_regs if g_pos is not set before
a0e372f0c49ac01faeaeb73a6e8f50e8ac615f34

@@ -2036,25 +2003,22 @@ void gdb_register_coprocessor(CPUState *cpu,
     }
 
     s = g_new0(GDBRegisterState, 1);
-    s->base_reg = last_reg;
+    s->base_reg = cpu->gdb_num_regs;
     s->num_regs = num_regs;
     s->get_reg = get_reg;
     s->set_reg = set_reg;
     s->xml = xml;
 
     /* Add to end of list.  */
-    last_reg += num_regs;
+    cpu->gdb_num_regs += num_regs;
     *p = s;
     if (g_pos) {
         if (g_pos != s->base_reg) {
             fprintf(stderr, "Error: Bad gdb register numbering for '%s'\n"
                     "Expected %d got %d\n", xml, g_pos, s->base_reg);
-        } else {
-            num_g_regs = last_reg;
         }
     }
 }

Re: [Qemu-devel] [PATCH] gdb: Fix gdb error

[Andreas Färber]

Am 12.08.2013 11:17, schrieb Aneesh Kumar K.V:
> Andreas Färber <afaerber@suse.de> writes:
>> Am 11.08.2013 20:14, schrieb Aneesh Kumar K.V:
>>> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
>>>
>>> Don't update the global register count if not requested.
>>> Without this patch a remote gdb session gives
>>>
>>> (gdb) target remote localhost:1234
>>> Remote debugging using localhost:1234
>>> Remote 'g' packet reply is too long:
>>> 0000000028000084c000000000ccba50c000000000c ...
>>> ....
>>> ...
>>> (gdb)
>>>
>>> This is a regression introduce by a0e372f0c49ac01faeaeb73a6e8f50e8ac615f34
>>>
>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>>
>> Thanks for tracking this down. I'm willing to include a variation in
>> today's pull to fix 1.6.0-rc3. However, did you find an explanation
>> *why* it needs to be like this?
> 
> IIUC our reply packet for 'g' contain more data becaue we ended up with
> larger cpu->gdb_num_regs. This only happens for archs that do a
> gdb_register_coprocessor with gpos == 0. The older code didn't update
> num_g_regs in that case. Not sure why we do like that
> 
>> I understand it is a revert to using the
>> static variable, updated to using the CPUClass field rather than the
>> previous preprocessor constant.
>>
> 
> I don't really like the patch. But I also don't know enough to fix this
> without using the static variable.  If you want me to try another
> version please send it across. I can easily reproduce this on PowerPC.

While it's always unfortunate to have a known breakage, we decided it's
too late/risky to address this for -rc3 today.

I put together a different patch that hopefully fixes the breakage while
avoiding to revert to static variables:
http://patchwork.ozlabs.org/patch/266594/

If this fixes things for you and doesn't break other things, we can get
it into qemu.git after Thursday [1] and backport it into 1.6.1.

Regards,
Andreas

[1] http://wiki.qemu.org/Planning/1.6

-- 
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg