Re: [PATCH v6 12/27] util: avoid repeated prefix on incremental qemu_log calls

On Wed, Feb 18, 2026 at 10:52:07AM +0100, Markus Armbruster wrote:
> Daniel P. Berrangé <[email protected]> writes:
> 
> > There are three general patterns to QEMU log output
> >
> >  1. Single complete message calls
> >
> >       qemu_log("Some message\n");
> >
> >  2. Direct use of fprintf
> >
> >       FILE *f = qemu_log_trylock()
> >       fprintf(f, "...");
> >       fprintf(f, "...");
> >       fprintf(f, "...\n");
> >       qemu_log_unlock(f)
> 
> Real code needs to check @f or risk crashing.  Shouldn't we show that
> here?

Yes, we should really.

> >  3. Mixed use of qemu_log_trylock/qemu_log()
> >
> >       FILE *f = qemu_log_trylock()
> >       qemu_log("....");
> >       qemu_log("....");
> >       qemu_log("....\n");
> >       qemu_log_unlock(f)
> >
> > When message prefixes are enabled, the timestamp will be
> > unconditionally emitted for all qemu_log() calls. This
> > works fine in the 1st case, and has no effect in the 2nd
> > case. In the 3rd case, however, we get the timestamp
> > printed over & over in each fragment.
> >
> > One can suggest that pattern (3) is pointless as it is
> > functionally identical to (2) but with extra indirection
> > and overhead. None the less we have a fair bit of code
> > that does this.
> >
> > The qemu_log() call itself is nothing more than a wrapper
> > which does pattern (2) with a single fprintf() call.
> 
> qemu_log_trylock()'s lock is recursive.  Worth a comment?  Not sure.

Sure.

> > Fixes: 012842c07552 (log: make '-msg timestamp=on' apply to all qemu_log usage)
> > Reported-by: Richard Henderson <[email protected]>
> > Signed-off-by: Daniel P. Berrangé <[email protected]>
> 
> Reviewed-by: Markus Armbruster <[email protected]>
> 

With regards,
Daniel
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