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Subject: Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get the selected hardware PMU conuter
From: pi3orama <pi3orama@163.com>
In-Reply-To: <20150717124538.GI19282@twins.programming.kicks-ass.net>
Date: Fri, 17 Jul 2015 20:57:00 +0800
Cc: "Wangnan (F)" <wangnan0@huawei.com>, kaixu xia <xiakaixu@huawei.com>,
        "ast@plumgrid.com" <ast@plumgrid.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "acme@kernel.org" <acme@kernel.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "masami.hiramatsu.pt@hitachi.com" <masami.hiramatsu.pt@hitachi.com>,
        "jolsa@kernel.org" <jolsa@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "hekuang@huawei.com" <hekuang@huawei.com>
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References: <1437129816-13176-6-git-send-email-xiakaixu@huawei.com> <20150717110541.GJ12596@twins.programming.kicks-ass.net> <55A8E703.70306@huawei.com> <20150717113924.GD19282@twins.programming.kicks-ass.net> <55A8EABE.1060308@huawei.com> <20150717115505.GF19282@twins.programming.kicks-ass.net> <20150717115615.GA18673@twins.programming.kicks-ass.net> <55A8EE83.3000708@huawei.com> <20150717121836.GH19282@twins.programming.kicks-ass.net> <55A8F4BF.3020902@huawei.com> <20150717124538.GI19282@twins.programming.kicks-ass.net>
To: Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
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> ?? 2015??7??17?գ?????8:45??Peter Zijlstra <peterz@infradead.org> д????
> 
>> On Fri, Jul 17, 2015 at 08:27:43PM +0800, Wangnan (F) wrote:
>> I think we can check the limitation in BPF program.
> 
> You typically do not want to rely on your program for correctness.

Sorry. NOT BPF program. What I want to express is bpf_read_pmu() function which is called from BPF program. User can put anything into the map during preparation but he or she gets only error code if the perf event he or she read from doesn't meet our restriction at runtime.

>> What about this:
>> 
>> event must on current CPU or must be on current process. If not,
>> bpf_read_pmu() should simply return an error.
> 
> OK, that's workable. That enforces the constraints outside of the
> program itself.
> 
>> With current design it is easy to implement, and users can still control
>> it through bpf map.
>> 
>> But what if we really want cross-cpu PMU accessing? Impossible?
> 
> Under the assumption that the eBPF program is called from tracing, and
> therefore from any context (task, softirq, irq and nmi), yes impossible.
> 
> You cannot do (synchronous) IPIs from either IRQ context or with IRQs
> disabled. And both are valid trace contexts.

What about software perf event? For example, tracepoints?

Thank you.
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