Messages in this thread | | | Date | Thu, 3 Sep 2009 23:58:10 +0530 | From | "K.Prasad" <> | Subject | Re: [Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware Breakpoint requests |
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On Wed, Sep 02, 2009 at 01:51:33AM +0200, Frederic Weisbecker wrote: > On Tue, Sep 01, 2009 at 12:08:45PM +0530, K.Prasad wrote: > > On Sat, Aug 29, 2009 at 03:41:07PM +0200, Ingo Molnar wrote: > > > > > > * K.Prasad <prasad@linux.vnet.ibm.com> wrote: > > > > > > > I am not sure if pmus can handle, (or want to handle) all the > > > > intricacies involved with the hw-breakpoint layer [...] > > > > > > Which are those intricacies? It's all rather straightforward > > > register scheduling and reservation stuff - which perfcounters > > > already solves in a very rich way. > > > > > > Ingo > > [edited] > > And post integration, in-kernel users like ptrace, kgdb* and xmon* > > which hitherto have interacted directly with the debug registers > > (through set_debugreg()/set_dabr()) should route their requests through the > > perf-layer. It is difficult to imagine ptrace's idempotent requests > > (through ptrace_<get><set>_debugreg()) having to pass through perf-layer > > (and becoming dependant on CONFIG_PERF_COUNTERS), not to mention the > > tricks required to synchronise signal generation timing with exception > > behaviour (especially on PPC64). > > * - Not converted to use hw-breakpoint layer yet > > > Actually, I see the perf layer here as a middle man between > > - the very hardware stuff (dr[0-467]) handling, reading, writing, updating > - the core API (register_kernel_breakpoint(), register_user_breakpoint() etc..) > > And this middle man can handle so much things on its own that the two above > gets utterly shrinked. > > Also the ptrace thing is tricky in itself, and that can't be helped easily. > Because of the direct writing to debug registers done by POKE_USR, > whatever the current breakpoint API with or without perf integration, we still > need subterfuges to carry it. >
The reverse-dependancy this would create over perf (CONFIG_PERF) for the hw-breakpoint layer is an undesirable side-effect, and gives rise to atleast two immediate questions:
- Handling of requests for hw-breakpoint from users like ptrace when CONFIG_PERF is not turned on - Managing 'register scheduling and reservation' on architectures where perf layer isn't ported. An inefficient way of handling this would be to retain the existing register allocation code of hw-breakpoint for such architectures - thereby artificially imposing arch-specific code into generic stuff.
A solution here would be to detach parts of perf layer's code that handle register scheduling and reservation (which I learn are in kernel/perf_counter.c) into a separate entity (outside the ambit of CONFIG_PERF) that can serve the needs of both hw-breakpoint and perf thereby eliminating the two issues enumerated above.
The tight coupling between the functions that perform register scheduling (in kernel/perf_counter.c) and perf's data structures is quite apparent and does suggest non-trivial amount of effort to detach them into a layer of its own.
However this might be quite necessary in order to balance between a desire to re-use the 'register scheduling and reservation' code of perf-layer while not running into issues as above.
This, along with the framework (described in the previous mail) to retain the hw-breakpoint's APIs + code interacting with debug registers (including exception handling) would be a good compromise.
Thanks, K.Prasad
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