Re: [patch 0/4] Port KVM-trace to tracepoints

From: Frank Ch. Eigler
Date: Tue Jul 22 2008 - 15:19:42 EST

Avi Kivity <avi@xxxxxxxxxxxx> writes:

> [...]
> kvm tracepoints are heavily tied into the implementation; and making
> them harder to write means we will have less information. In fact, I
> am contemplating moving in another direction (when looking at the
> pgprintk()s scattered around arch/x86/kvm/mmu.c:
> kvm_trace("pfentry", "page_fault entry addr %lx error code %x\n",
> cr2, error_code);
> Unlike printk()s, no actual formatting would occur during runtime.

Have you considered using trace_mark() directly - eliminating the
KVM_TRACEN() middlemen?

> Instead, at initialization time all the strings would be parsed into
> a data structure that describes the data types, and the runtime
> would simply consult this structure and copy the arguments into
> trace records. User space would also be able to pull this structure
> and so recreate the formatted string.

If one really wanted to, one could build such a mechanism on top of
marker-based callbacks.

> The advantages I see to this are:
> - easy to add traces; the most important advantage
> - when the code changes, obsolete traces are completely removed
> - good performance


> - no need to have a formats file in userspace (which is tied to the
> kernel version)

OTOH, you'd have the kernel collecting compact binary records
containing just the parameters, which are at least as tied to kernel

> - can also send printk()s along, for synchronization with other kvm
> and kernel events

Ditto. It is elementary to attach a printk-generating marker callback.

- FChE
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