[Qemu-devel] Re: [RFC v3][PATCH][Tracing] Fix build errors for target i386-linux-user

[Prerna Saxena <prerna@linux.vnet.ibm.com>]

On 07/08/2010 02:50 PM, Stefan Hajnoczi wrote:
> On Thu, Jul 08, 2010 at 10:58:58AM +0530, Prerna Saxena wrote:
>> [PATCH] Separate monitor command handler interfaces and tracing internals.
>>
>>
>> Signed-off-by: Prerna Saxena<prerna@linux.vnet.ibm.com>
>> ---
>>   monitor.c     |   23 +++++++++++++++++++++++
>>   simpletrace.c |   51 +++++++++++++++++++++++++++++----------------------
>>   tracetool     |    7 +++++++
>>   3 files changed, 59 insertions(+), 22 deletions(-)
>>
>> diff --git a/monitor.c b/monitor.c
>> index 433a3ec..1f89938 100644
>> --- a/monitor.c
>> +++ b/monitor.c
>> @@ -540,6 +540,29 @@ static void do_change_trace_event_state(Monitor *mon, const QDict *qdict)
>>       bool new_state = qdict_get_bool(qdict, "option");
>>       change_trace_event_state(tp_name, new_state);
>>   }
>> +
>> +void do_info_trace(Monitor *mon)
>> +{
>> +    unsigned int i;
>> +    char rec[MAX_TRACE_STR_LEN];
>> +    unsigned int trace_idx = get_trace_idx();
>> +
>> +    for (i = 0; i<  trace_idx ; i++) {
>> +        if (format_trace_string(i, rec)) {
>> +            monitor_printf(mon, rec);
>> +        }
>> +    }
>> +}
>> +
>> +void do_info_all_trace_events(Monitor *mon)
>> +{
>> +    unsigned int i;
>> +
>> +    for (i = 0; i<  NR_TRACE_EVENTS; i++) {
>> +        monitor_printf(mon, "%s [Event ID %u] : state %u\n",
>> +                                trace_list[i].tp_name, i, trace_list[i].state);
>> +    }
>> +}
>>   #endif
>>
>>   static void user_monitor_complete(void *opaque, QObject *ret_data)
>> diff --git a/simpletrace.c b/simpletrace.c
>> index 57c41fc..c7b1e7e 100644
>> --- a/simpletrace.c
>> +++ b/simpletrace.c
>> @@ -1,8 +1,8 @@
>>   #include<stdlib.h>
>>   #include<stdio.h>
>> -#include "monitor.h"
>>   #include "trace.h"
>>
>> +/* Remember to update TRACE_REC_SIZE when changing TraceRecord structure */
>
> I can't see TRACE_REC_SIZE anywhere else in this patch.

Oops. This comment must go. The connotation was for MAX_TRACE_STR_LEN to 
be large enough to hold the formatted string, but I'm not sure if there 
is a way to test that.

>
>>   typedef struct {
>>       unsigned long event;
>>       unsigned long x1;
>> @@ -69,27 +69,6 @@ void trace5(TraceEventID event, unsigned long x1, unsigned long x2, unsigned lon
>>       trace(event, x1, x2, x3, x4, x5);
>>   }
>>
>> -void do_info_trace(Monitor *mon)
>> -{
>> -    unsigned int i;
>> -
>> -    for (i = 0; i<  trace_idx ; i++) {
>> -        monitor_printf(mon, "Event %lu : %lx %lx %lx %lx %lx\n",
>> -                          trace_buf[i].event, trace_buf[i].x1, trace_buf[i].x2,
>> -                            trace_buf[i].x3, trace_buf[i].x4, trace_buf[i].x5);
>> -    }
>> -}
>> -
>> -void do_info_all_trace_events(Monitor *mon)
>> -{
>> -    unsigned int i;
>> -
>> -    for (i = 0; i<  NR_TRACE_EVENTS; i++) {
>> -        monitor_printf(mon, "%s [Event ID %u] : state %u\n",
>> -                                trace_list[i].tp_name, i, trace_list[i].state);
>> -    }
>> -}
>> -
>>   static TraceEvent* find_trace_event_by_name(const char *tname)
>>   {
>>       unsigned int i;
>> @@ -115,3 +94,31 @@ void change_trace_event_state(const char *tname, bool tstate)
>>           tp->state = tstate;
>>       }
>>   }
>> +
>> +/**
>> + * Return the current trace index.
>> + *
>> + */
>> +unsigned int get_trace_idx(void)
>> +{
>> +    return trace_idx;
>> +}
>
> format_trace_string() returns NULL if the index is beyond the last valid
> trace record.  monitor.c doesn't need to know how many trace records
> there are ahead of time, it can just keep printing until it gets NULL.
> I don't feel strongly about this but wanted to mention it.

format_trace_string() returns NULL when the index passed exceeds the 
size of trace buffer. This function is meant for printing current 
contents of trace buffer, which may be less than the entire buffer size.

>
>> +
>> +/**
>> + * returns formatted TraceRecord at a given index in the trace buffer.
>> + * FORMAT : "Event %lu : %lx %lx %lx %lx %lx\n"
>> + *
>> + * @idx : index in the buffer for which trace record is returned.
>> + * @trace_str : output string passed.
>> + */
>> +char* format_trace_string(unsigned int idx, char trace_str[])
>> +{
>> +    TraceRecord rec;
>> +    if (idx>= TRACE_BUF_LEN || sizeof(trace_str)>= MAX_TRACE_STR_LEN) {
>
> sizeof(trace_str) == sizeof(char *), not the size of the caller's array in bytes.

Hmm, I'll need to scrap off this check.

>
> The fixed size limit can be eliminated using asprintf(3), which
> allocates a string of the right size while doing the string formatting.
> The caller of format_trace_string() is then responsible for freeing the
> string when they are done with it.
>

I am somehow reluctant to allocate memory here and free it somewhere 
else. Calls for memory leaks quite easily in case it gets missed.
I'd rather use stack-allocated arrays that clean up after the call to 
the handler is done.

>> +        return NULL;
>> +    }
>> +    rec = trace_buf[idx];
>
> Is it necessary to copy the trace record here?

In my understanding, this would run in the context of monitor handlers, 
which are executed in a separate thread other than the main qemu 
execution loop. Since sprintf() is a longer operation, considering the 
trace_idx might get incremented in the meantime -- I had thought copying 
the TraceRecord would be achieved more quickly with lesser probability 
of index slipping away. Might be an over-optimization -- pls correct me 
if I'm wrong :-)

>
>> +    sprintf(&trace_str[0], "Event %lu : %lx %lx %lx %lx %lx\n",
>> +                            rec.event, rec.x1, rec.x2, rec.x3, rec.x4, rec.x5);
>> +    return&trace_str[0];
>> +}
>> diff --git a/tracetool b/tracetool
>> index c77280d..b7a0499 100755
>> --- a/tracetool
>> +++ b/tracetool
>> @@ -125,6 +125,11 @@ typedef struct {
>>       bool state;
>>   } TraceEvent;
>>
>> +/* Max size of trace string to be displayed via the monitor.
>> + * Format : "Event %lu : %lx %lx %lx %lx %lx\n"
>> + */
>> +#define MAX_TRACE_STR_LEN 100
>> +
>>   void trace1(TraceEventID event, unsigned long x1);
>>   void trace2(TraceEventID event, unsigned long x1, unsigned long x2);
>>   void trace3(TraceEventID event, unsigned long x1, unsigned long x2, unsigned long x3);
>> @@ -133,6 +138,8 @@ void trace5(TraceEventID event, unsigned long x1, unsigned long x2, unsigned lon
>>   void do_info_trace(Monitor *mon);
>>   void do_info_all_trace_events(Monitor *mon);
>>   void change_trace_event_state(const char *tname, bool tstate);
>> +unsigned int get_trace_idx(void);
>> +char* format_trace_string(unsigned int idx, char *trace_str);
>
> I think we need to choose a prefix like simpletrace_*() or something
> more concise (but not "strace_" because it's too confusing).  Other
> subsystems tend to do this: pci_*(), ram_*(), etc.
>

Agree, it is useful. However, simpletrace_ is too big for a prefix. 
Maybe st_ works, though it is slightly on the ambiguous side ?

> Perhaps let's do it as a separate patch to fix up all of the simple
> trace backend.
>

Will do.

Thanks,
-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India