Re: [RFC 0/2] KVM: Fix kvm_exit trace event format

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Wed, Jul 27, 2011 at 02:47:20PM +0300, Avi Kivity wrote:
> On 07/22/2011 02:46 PM, Stefan Hajnoczi wrote:
> >In practice both perf and trace-cmd are not happy with the new exit_reason
> >formatting expression (omitting the details and split across lines for easy
> >email reading here):
> >
> >print fmt: "reason %s rip 0x%lx info %llx %llx",
> >            (REC->isa == 1) ?
> >            __print_symbolic(REC->exit_reason, { 0, "EXCEPTION_NMI" }, ...) :
> >            __print_symbolic(REC->exit_reason, { 0x000, "read_cr0" }, ...),
> >            REC->guest_rip, REC->info1, REC->info2
> >
> >perf script says:
> >
> >   Warning: Error: expected type 5 but read 4
> >   Warning: Error: expected type 5 but read 0
> >   Warning: unknown op '}'
> >
> >kvm  2696 [001]   289.850941: kvm_exit: EVENT 'kvm_exit' FAILED TO PARSE
> >
> >trace-cmd says:
> >
> >   Error: expected type 5 but read 4
> >   Error: expected type 5 but read 0
> >   failed to read event print fmt for kvm_exit
> >
> >kvm-2696  [000]  1451.564092: kvm_exit: [FAILED TO PARSE] exit_reason=44 guest_rip=0xc01151a8 isa=1 info1=4272 info2=0
> >
> >I'd really like to make perf and trace-cmd just work with kvm:kvm_exit.  Any
> >suggestions other than improving the parsers in the respective tools?
> 
> trace-cmd has a plugin API (and a kvm plugin) that you can use for
> this, and I expect perf either does as well or can make use of one.
> I don't think it's useful to complicate the print format description
> further to support our weird use case.

The problem with relying on a plugin is that every userspace application
(currently two that I am aware of: perf and trace-cmd) has to add
special-case kvm code to work around our crappy format.

With these patches we at least get back to an event format that is
self-describing - no plugin necessary.

However, since the event format parser in perf and trace-cmd is not
powerful enough to handle the ternary operator together with
__print_symbolic() I guess the parsers need to be improved.  It's not a
special-case hack for kvm.

If I have the time I'll try to make those parser improvements.

Stefan