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

[Gerhard Wiesinger]

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