[PATCH v2 0/2] Quality of life patches for qemu coroutine debugging

[PATCH v2 0/2] Quality of life patches for qemu coroutine debugging

[Maxim Levitsky]

These are two patches that improve a bit the qemu gdb scripts in regard to
coroutines.

First patch fixes a bug in 'qemu coroutine' in regard to usage on non topmost
stack frame.

Second patch implements 'qemu bt' as Stefan suggested.

V2, because after hitting 'send' I noticed a few bugs:
   1. Switch to try/catch around bt, so that when regular bt fails, we
      print the same message as original bt.
   2. Print the correct co-routine address

Best regards,
        Maxim Levitsky

Maxim Levitsky (2):
  scripts/gdb: fix 'qemu coroutine' when users selects a non topmost
    stack frame
  scripts/gdb: implement 'qemu bt'

 scripts/qemu-gdb.py          |  1 +
 scripts/qemugdb/coroutine.py | 35 ++++++++++++++++++++++++++++++++++-
 2 files changed, 35 insertions(+), 1 deletion(-)

-- 
2.26.2

[PATCH v2 1/2] scripts/gdb: fix 'qemu coroutine' when users selects a non topmost stack frame

[Maxim Levitsky]

The code that dumps the stack frame works like that:
* save current registers
* overwrite current registers (including rip/rsp) with coroutine snapshot
  in the jmpbuf
* print backtrace
* restore the saved registers.

If the user has currently selected a non topmost stack frame in gdb,
the above code will still restore the selected frame registers,
but the gdb will then lose the selected frame index, which makes it impossible
to switch back to frame 0, to continue debugging the executable.

Therefore switch temporarily to the topmost frame of the stack
for the above code.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 scripts/qemugdb/coroutine.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/scripts/qemugdb/coroutine.py b/scripts/qemugdb/coroutine.py
index db61389022..e1399211e6 100644
--- a/scripts/qemugdb/coroutine.py
+++ b/scripts/qemugdb/coroutine.py
@@ -70,6 +70,11 @@ def bt_jmpbuf(jmpbuf):
     regs = get_jmpbuf_regs(jmpbuf)
     old = dict()
 
+    # remember current stack frame and select the topmost
+    # so that register modifications don't wreck it
+    selected_frame = gdb.selected_frame()
+    gdb.newest_frame().select()
+
     for i in regs:
         old[i] = gdb.parse_and_eval('(uint64_t)$%s' % i)
 
@@ -81,6 +86,8 @@ def bt_jmpbuf(jmpbuf):
     for i in regs:
         gdb.execute('set $%s = %s' % (i, old[i]))
 
+    selected_frame.select()
+
 def coroutine_to_jmpbuf(co):
     coroutine_pointer = co.cast(gdb.lookup_type('CoroutineUContext').pointer())
     return coroutine_pointer['env']['__jmpbuf']
-- 
2.26.2

[PATCH v2 2/2] scripts/gdb: implement 'qemu bt'

[Maxim Levitsky]

This script first runs the regular gdb's 'bt' command, and then if we are in a
coroutine it prints the coroutines backtraces in the order in which they
were called.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 scripts/qemu-gdb.py          |  1 +
 scripts/qemugdb/coroutine.py | 28 +++++++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/scripts/qemu-gdb.py b/scripts/qemu-gdb.py
index e0bfa7b5a4..4d2a9f6c43 100644
--- a/scripts/qemu-gdb.py
+++ b/scripts/qemu-gdb.py
@@ -40,6 +40,7 @@ timers.TimersCommand()
 
 coroutine.CoroutineSPFunction()
 coroutine.CoroutinePCFunction()
+coroutine.CoroutineBt()
 
 # Default to silently passing through SIGUSR1, because QEMU sends it
 # to itself a lot.
diff --git a/scripts/qemugdb/coroutine.py b/scripts/qemugdb/coroutine.py
index e1399211e6..7db46d4b68 100644
--- a/scripts/qemugdb/coroutine.py
+++ b/scripts/qemugdb/coroutine.py
@@ -88,8 +88,11 @@ def bt_jmpbuf(jmpbuf):
 
     selected_frame.select()
 
+def co_cast(co):
+    return co.cast(gdb.lookup_type('CoroutineUContext').pointer())
+
 def coroutine_to_jmpbuf(co):
-    coroutine_pointer = co.cast(gdb.lookup_type('CoroutineUContext').pointer())
+    coroutine_pointer = co_cast(co)
     return coroutine_pointer['env']['__jmpbuf']
 
 
@@ -107,6 +110,29 @@ class CoroutineCommand(gdb.Command):
 
         bt_jmpbuf(coroutine_to_jmpbuf(gdb.parse_and_eval(argv[0])))
 
+class CoroutineBt(gdb.Command):
+    '''Display backtrace including coroutine switches'''
+    def __init__(self):
+        gdb.Command.__init__(self, 'qemu bt', gdb.COMMAND_STACK,
+                             gdb.COMPLETE_NONE)
+
+    def invoke(self, arg, from_tty):
+
+        gdb.execute("bt")
+
+        if gdb.parse_and_eval("qemu_in_coroutine()") == False:
+            return
+
+        co_ptr = gdb.parse_and_eval("qemu_coroutine_self()")
+
+        while True:
+            co = co_cast(co_ptr)
+            co_ptr = co["base"]["caller"]
+            if co_ptr == 0:
+                break
+            gdb.write("Coroutine at " + str(co_ptr) + ":\n")
+            bt_jmpbuf(coroutine_to_jmpbuf(co_ptr))
+
 class CoroutineSPFunction(gdb.Function):
     def __init__(self):
         gdb.Function.__init__(self, 'qemu_coroutine_sp')
-- 
2.26.2

Re: [PATCH v2 0/2] Quality of life patches for qemu coroutine debugging

[Maxim Levitsky]

On Thu, 2020-12-17 at 17:54 +0200, Maxim Levitsky wrote:
> These are two patches that improve a bit the qemu gdb scripts in regard to
> coroutines.
> 
> First patch fixes a bug in 'qemu coroutine' in regard to usage on non topmost
> stack frame.
> 
> Second patch implements 'qemu bt' as Stefan suggested.
> 
> V2, because after hitting 'send' I noticed a few bugs:
>    1. Switch to try/catch around bt, so that when regular bt fails, we
>       print the same message as original bt.
>    2. Print the correct co-routine address
> 
> Best regards,
>         Maxim Levitsky
> 
> Maxim Levitsky (2):
>   scripts/gdb: fix 'qemu coroutine' when users selects a non topmost
>     stack frame
>   scripts/gdb: implement 'qemu bt'
> 
>  scripts/qemu-gdb.py          |  1 +
>  scripts/qemugdb/coroutine.py | 35 ++++++++++++++++++++++++++++++++++-
>  2 files changed, 35 insertions(+), 1 deletion(-)
> 
> -- 
> 2.26.2
> 
> 
> 
Any update on this?
Best regards,
	Maxim Levitsky

Re: [PATCH v2 0/2] Quality of life patches for qemu coroutine debugging

[Paolo Bonzini]

On 07/01/21 11:24, Maxim Levitsky wrote:
> On Thu, 2020-12-17 at 17:54 +0200, Maxim Levitsky wrote:
>> These are two patches that improve a bit the qemu gdb scripts in regard to
>> coroutines.
>>
>> First patch fixes a bug in 'qemu coroutine' in regard to usage on non topmost
>> stack frame.
>>
>> Second patch implements 'qemu bt' as Stefan suggested.
>>
>> V2, because after hitting 'send' I noticed a few bugs:
>>     1. Switch to try/catch around bt, so that when regular bt fails, we
>>        print the same message as original bt.
>>     2. Print the correct co-routine address
>>
>> Best regards,
>>          Maxim Levitsky
>>
>> Maxim Levitsky (2):
>>    scripts/gdb: fix 'qemu coroutine' when users selects a non topmost
>>      stack frame
>>    scripts/gdb: implement 'qemu bt'
>>
>>   scripts/qemu-gdb.py          |  1 +
>>   scripts/qemugdb/coroutine.py | 35 ++++++++++++++++++++++++++++++++++-
>>   2 files changed, 35 insertions(+), 1 deletion(-)
>>
>> -- 
>> 2.26.2
>>
>>
>>
> Any update on this?
> Best regards,
> 	Maxim Levitsky
> 
> 

I'll queue them myself, thanks.

Paolo

Re: [PATCH v2 0/2] Quality of life patches for qemu coroutine debugging

[Maxim Levitsky]

On Thu, 2021-01-07 at 19:05 +0100, Paolo Bonzini wrote:
> On 07/01/21 11:24, Maxim Levitsky wrote:
> > On Thu, 2020-12-17 at 17:54 +0200, Maxim Levitsky wrote:
> > > These are two patches that improve a bit the qemu gdb scripts in regard to
> > > coroutines.
> > > 
> > > First patch fixes a bug in 'qemu coroutine' in regard to usage on non topmost
> > > stack frame.
> > > 
> > > Second patch implements 'qemu bt' as Stefan suggested.
> > > 
> > > V2, because after hitting 'send' I noticed a few bugs:
> > >     1. Switch to try/catch around bt, so that when regular bt fails, we
> > >        print the same message as original bt.
Note that I forgot to 'git add' this bit, but it is only a cosmetic thing,
so I'll guess I'll just leave it like that.

> > >     2. Print the correct co-routine address
> > > 
> > > Best regards,
> > >          Maxim Levitsky
> > > 
> > > Maxim Levitsky (2):
> > >    scripts/gdb: fix 'qemu coroutine' when users selects a non topmost
> > >      stack frame
> > >    scripts/gdb: implement 'qemu bt'
> > > 
> > >   scripts/qemu-gdb.py          |  1 +
> > >   scripts/qemugdb/coroutine.py | 35 ++++++++++++++++++++++++++++++++++-
> > >   2 files changed, 35 insertions(+), 1 deletion(-)
> > > 
> > > -- 
> > > 2.26.2
> > > 
> > > 
> > > 
> > Any update on this?
> > Best regards,
> > 	Maxim Levitsky
> > 
> > 
> 
> I'll queue them myself, thanks.

Thanks!

Best regards,
	Maxim Levitsky
> 
> Paolo
>