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

Avi Kivity
2008-07-22T19:31:55Z

Frank Ch. Eigler wrote:
> Avi Kivity <avi@qumranet.com> 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?
>
>

Eliminating KVM_TRACEN -- yes. There are too many of them, they aren't
type-aware, and they're in uppercase.

Using trace_mark() directly -- looking at it, seems to fit the
requirements exactly. Should have looked at it earlier. Is there a way
to get a list of all markers?

Perhaps the kvmtrace marker->relay integration should be made a marker
feature, since there is nothing specific to kvm in it.

>> 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.
>
>

One does want to.

>> - 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
> version.
>
>

Yes, but the userspace side would collect the format strings as well
(just once) and could put them in the same file. The aggregation is
portable across kernel versions.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.