Re: [patch 4/4] KVM-trace port to tracepoints

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Jan Kiszka (jan.kiszka@siemens.com) wrote:
> Mathieu Desnoyers wrote:
> > Port/cleanup of KVM-trace to tracepoints.
> > 
> > Tracepoints allow dormat instrumentation, like the kernel markers, but also
> > allows to describe the trace points in global headers so they can be easily
> > managed. They also do not use format strings.
> > 
> > Anything that would involve an action (dereference a pointer, vmcs read, ...)
> > only required when tracing is placed in the probes created in kvm_trace.c
> > 
> > This patch depends on the "Tracepoints" patch.
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > CC: 'Peter Zijlstra' <peterz@infradead.org>
> > CC: 'Feng(Eric) Liu' <eric.e.liu@intel.com>
> > CC: Avi Kivity <avi@qumranet.com>
> > CC: kvm@vger.kernel.org
> > ---
> >  arch/x86/kvm/vmx.c   |   38 ++---
> >  arch/x86/kvm/x86.c   |   43 ++----
> >  include/trace/kvm.h  |   83 ++++++++++++
> >  virt/kvm/kvm_trace.c |  336 +++++++++++++++++++++++++++++++++++++++++++--------
> >  4 files changed, 398 insertions(+), 102 deletions(-)
> 
> Is it a specific property of KVM-trace that causes this LOC blow-up? Or
> is this a generic side-effect of tracepoints?
> 
> [ Hmm, hope I didn't missed too much of the tracepoint discussion... ]
> 

This LOC blow-up is caused by the creation of one probe per
instrumentation site. So instead of placing the argument setup of
everything that goes in the trace (0 to 5 u32 arguments) in the kvm
code, it can be placed separately in a probe object, which could
eventually be a dynamically loadable module.

The primary objective of tracepoints is to make sure the kernel code
does not become harder to read because of added instrumentation and to
provide type-checking at compile-time without needing to put format
strings into the kernel code, which, to some, looks like debugging code.
The other aspect it try to address is maintainability of trace points :
it's much easier to look at all the prototype definitions in
include/trace/*.h and to manage them (and external tracers which would
like to connect on those points) than to try to figure out in which C
files tracing statements has been hidden. We can think of it as a
standard tracing API providing a more or less stable list of kernel
tracepoints.

So, while KVMTRACE_?D() statements suits closely kvm-trace
specificities, it's useless to impose constraints such as splitting
unsigned longs into two u32 for tracers which can support a wider
variety of data types.

After refactoring the patch to put the probes in arch/x86/kvm, the
result is :

 arch/x86/kvm/Makefile           |    1
 arch/x86/kvm/kvm_trace_probes.c |  251 ++++++++++++++++++++++++++++++++++++++++
 arch/x86/kvm/vmx.c              |   38 ++----
 arch/x86/kvm/x86.c              |   43 ++----
 include/asm-x86/kvm_host.h      |    8 +
 include/trace/kvm.h             |   83 +++++++++++++
 virt/kvm/kvm_trace.c            |   93 ++++++--------
 7 files changed, 414 insertions(+), 103 deletions(-)

So actually, is it better to have less LOC which looks like this :

        KVMTRACE_5D(CPUID, vcpu, function,
                    (u32)kvm_register_read(vcpu, VCPU_REGS_RAX),
                    (u32)kvm_register_read(vcpu, VCPU_REGS_RBX),
                    (u32)kvm_register_read(vcpu, VCPU_REGS_RCX),
                    (u32)kvm_register_read(vcpu, VCPU_REGS_RDX), handler);


or more LOC looking like this :

include/trace/kvm.h:
DEFINE_TRACE(kvm_cpuid,
        TPPROTO(struct kvm_vcpu *vcpu, u32 function),
        TPARGS(vcpu, function));

arch/x86/kvm/x86.c:
        trace_kvm_cpuid(vcpu, function);

arch/x86/kvm/kvm_trace_probes.c:
static void probe_kvm_cpuid(struct kvm_vcpu *vcpu, u32 function)
{
        kvm_add_trace(KVM_TRC_CPUID, vcpu, 5,
                (u32 []){ function,
                        (u32)kvm_register_read(vcpu, VCPU_REGS_RAX),
                        (u32)kvm_register_read(vcpu, VCPU_REGS_RBX),
                        (u32)kvm_register_read(vcpu, VCPU_REGS_RCX),
                        (u32)kvm_register_read(vcpu, VCPU_REGS_RDX) });
}

int register_kvm_tracepoints(void)
{
   ...
   ret = register_trace_kvm_cpuid(probe_kvm_cpuid);
   WARN_ON(ret);
   ...
}

void unregister_kvm_tracepoints(void)
{
   ...
   unregister_trace_kvm_cpuid(probe_kvm_cpuid);
   ...
}

?

Notice that only a single line of code is inserted to the kernel code,
while all the rest sits outsite in a separated probe module. So I think
it's very important to distinguish between LOC which impair kernel code
readability and LOC which sit in their own sandbox.

Mathieu

> Jan
> 
> -- 
> Siemens AG, Corporate Technology, CT SE 2
> Corporate Competence Center Embedded Linux

-- 
Mathieu Desnoyers
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