RE: [patch 0/2] x86, ptrace: support for branch trace store(BTS)

[Metzger, Markus T]

>-----Original Message-----
>From: Andi Kleen [mailto:ak@xxxxxxx] 
>Sent: Montag, 3. Dezember 2007 17:22

>> What did you have in mind when you asked for kernel-mode support?
>
>I asked about that earlier too and I would like to see per CPU traces
>for ring 0 with some way to dump that on crashes or on user trigger.

I will split it into two layers:
- a lower layer that does the DS/BTS configuration
- a higher layer that provides the ptrace interface

The lower layer works on a parameter DS pointer. Unfortunately,
this will be an unsigned long, since we have different DS/BTS layout
for different architectures.

The higher layer uses the lower layer to do the real work. It stores
a per-thread DS pointer in the thread_struct.

Kernel tracing would allocate a per_cpu array of DS pointers
and use the lower layer for all the real work. I would leave it
to someone who is more familiar with the kgdb patch, if that is OK.


What's left is proper resource managememt. The kernel use of
DS_SAVE_AREA
and DEBUGCTL conflicts with the ptrace use of those MSR's. As far as I
can
tell, this is missing for all MSR's. The single/block_stepping support 
certainly uses an optimistic approach.

I guess this is a certain amount of work. It needs the idea of an
'owner'
of such an MSR, some means to acquire and release it, a global
allocation 
order, some protection from others who simply try to use it. It needs to
be on register granularity for some; on bit granularity for others. It
needs to be global for some; per-thread for others. And it probably
needs 
a lot of error handling code in a lot of different areas.

I would hope that utrace is tackling some or all of these problems.
I would therefore stay optimistic in this patch and rework it once
utrace is in and provides a proper framework.
This seems to be the approach that the new block-stepping support takes.


thanks and regards,
markus.
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