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
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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