Re: Another Obsolete Fix me in trace.h?

Radim Krčmář
2014-11-24T21:00:01Z

2014-11-24 11:40+0100, Jan Kiszka:
> On 2014-11-24 11:12, Paolo Bonzini wrote:
> > On 24/11/2014 05:36, nick wrote:
> >> Greetings Again Gleb and others,
> >> I am assuming in the code I am pasting below the fix me is obsolete now and I can remove it. :)
> >> Cheers Nick
> >> TP_printk("%s (0x%x)",
> >> __print_symbolic(__entry->exception, kvm_trace_sym_exc),
> >> /* FIXME: don't print error_code if not present */
> >> __entry->has_error ? __entry->error_code : 0)
> >> );
> >>
> >
> > No, it's not obsolete, the idea is to print only
> >
> > %s
> >
> > instead of
> >
> > %s (0x%x)
> >
> > if __entry->has_error is false. I don't know the trace API well enough
> > to know if that is possible.
>
> Last time I ran across such a scenario, it was not feasible and
> essentially required separate tracepoints. But maybe Steven knows a trick.

The format string has to be a string literal[1]; we could change it to
allow expressions[2], but what we want is almost possible through a
direct call to trace_seq_printf()[3].

The raw result would look like

#define __print(fmt, args...) ({ \
const char *buf_start = trace_seq_buffer_ptr(p); \
trace_seq_printf(p, fmt, args); \
trace_seq_putc(p, '\0'); \
buf_start; \
})

TP_printk("%s%s", [...],
__entry->has_error ? __print("(0x%x)", __entry->error_code) : "")

and would be acceptable if something __print-like made it into a ftrace
helper[4]. (Userspace won't be able to nicely print it otherwise.)

---
1: #define TP_printk(fmt, args...) fmt "\n", args
2: TP_printk(__entry->has_error ? "%s (0x%x)" : "%s", [...]
3: Already in scsi_dispatch_cmd_start or kvm_mmu_get_page tracepoints.
4: Like __print_hex or print_symbolic.