Re: [Qemu-devel] [PATCH] block.c, block/vmdk.c: Fixed major bug in VMDK WRITE and READ handling - FIXES DATA CORRUPTION

[Gerhard Wiesinger <lists@wiesinger.com>]

On 10.11.2012 09:55, Paolo Bonzini wrote:
> Il 10/11/2012 09:30, Gerhard Wiesinger ha scritto:
>>>> 2.) Added debug code to block.c and to block/vmdk.c to verify
>>>> correctness
>>> Same here.  Also, please use the tracing infrastructure---a lot of the
>>> debug
>>> messages you're adding, though not all, are in fact already available
>>> (not
>>> saying the others aren't useful!)
>> Any chance that the patch with debug code only (after some cleaning)
>> would be accepted (other modules do debug logging, too)?
>> I  don't like to do useless work.
>> Tracing infrastructure is quite limited to function calls only (as far
>> as I saw).
> No, tracing infrastructure uses function calls for tracing (messages go
> into trace-events) but you can apply it to everything you want.  Use the
> stderr backend to debug it.

Tracing is a good thing for normal behavior but the major limitation is 
that a function call must be involved. But for deep debugging one needs 
a lot of more messages than function calls are available.

Of course every DPRINTF line could be made in a function call but IHMO 
this introduces unnecessary overhead in performance.

So how to proceed further, some options:
1.) Add additional function calls for each DPRINTF statement?
2.) Add just plain DPRINTF statements?
3.) Or a mixture of both: on function call boundaries use Tracing, in 
function debug info use DPRINTF?
4.) Refactor code that always function calls are involved?

Example:
static void traceing_func(int mul)
{
     // Do nothing here
}

// Just some dummy useless function doing illustration
static int addandmultiply(int arg1, int arg2)
{
     int mul = 0;
     int sum = arg1 + arg2;
     DPRINTF("....", arg1, arg2); // this one can be handled by tracing 
infrastructure
     DPRINTF("....", sum); // this one can't be done with tracing
     if (sum == 0)
     {
             DPRINTF("sum=0"); // this one can't be done with tracing
         return;
     }
     mul = arg1 * arg2;
     DPRINTF(...., mul);  // this one can't be done with tracing (except 
with a introduced tracing function)
     traceing_func(mul); // Introduce tracing function, performance penalty
     return sum + mul;
}

So how to proceed further?

Ciao,
Gerhard