Re: [PATCH] trace "exit to userspace" event

[Gleb Natapov <gleb@redhat.com>]

On Thu, Oct 14, 2010 at 01:32:14PM +0200, Avi Kivity wrote:
>  On 10/14/2010 01:28 PM, Gleb Natapov wrote:
> >On Thu, Oct 14, 2010 at 01:11:20PM +0200, Avi Kivity wrote:
> >>   On 10/14/2010 12:29 PM, Gleb Natapov wrote:
> >>  >On Thu, Oct 14, 2010 at 12:27:15PM +0200, Avi Kivity wrote:
> >>  >>    On 10/10/2010 05:46 PM, Gleb Natapov wrote:
> >>  >>   >>
> >>  >>   >>    We should log both errno and exit_reason.  If we want to be clever,
> >>  >>   >>    we can display strerror(errno) if it's nonzero, and exit_reason
> >>  >>   >>    otherwise (easy to do in a trace-cmd plugin).
> >>  >>   >>
> >>  >>   >For starters we should remove KVM_EXIT_INTR exit reason. Looking into
> >>  >>   >qemu-kvm history it was never used and there is at least one code path
> >>  >>   >that returns -EINTR and does not set KVM_EXIT_INTR, so exit_reason field
> >>  >>   >contains stale info on exit.
> >>  >>   >
> >>  >>
> >>  >>   The two issues are unrelated.
> >>  >>
> >>  >So what do you propose? I see no issue with my original patch.
> >>
> >>  Record both errno and exit_reason.  While they're never both valid
> >>  at the same time, they're both necessary.
> >>
> >If they can't be valid at the same time, why not record one of them?
> 
> If you record just one, you don't know if the other one happened.
> 
I mean to record type/value. So it can be (type error/value -EINVAL) or
(type exit/value HALT). The goal is to not print non-relevant info in
ftrace, but we can do the same with recording errno and exit_reason and
show only exit_reason is errno>=0 or errno otherwise.

> >The
> >one that happened? Also any error other then -EINTR will cause qemu to
> >stop, so it is not very interesting. And ioctl return value can be
> >traced by strace anyway.
> 
> You can't correlate it with ftrace.
> 
True. But given that the only interesting error code in -EINTR I do not
know if this is useful, but potentially may generate a lot of events in
ftrace. Sometimes too much info is almost as bad as not enough and if we
will use the same event for both exit_reason and errno it will be
impossible to enable one without the other.

--
			Gleb.