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Date:   Tue, 8 Jan 2019 19:31:27 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        acme@kernel.org, ast@kernel.org, daniel@iogearbox.net,
        kernel-team@fb.com
Subject: Re: [PATCH v5 perf, bpf-next 1/7] perf, bpf: Introduce
 PERF_RECORD_KSYMBOL
Message-ID: <20190108183127.GB30894@hirez.programming.kicks-ass.net>
References: <20181220182904.4193196-1-songliubraving@fb.com>
 <20181220182904.4193196-2-songliubraving@fb.com>
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On Thu, Dec 20, 2018 at 10:28:58AM -0800, Song Liu wrote:

> @@ -648,11 +649,18 @@ struct perf_event_mmap_page {
>   *   PERF_RECORD_MISC_COMM_EXEC  - PERF_RECORD_COMM event
>   *   PERF_RECORD_MISC_FORK_EXEC  - PERF_RECORD_FORK event (perf internal)
>   *   PERF_RECORD_MISC_SWITCH_OUT - PERF_RECORD_SWITCH* events
> + *   PERF_RECORD_MISC_KSYMBOL_*  - PERF_RECORD_KSYMBOL event
>   */
>  #define PERF_RECORD_MISC_MMAP_DATA		(1 << 13)
>  #define PERF_RECORD_MISC_COMM_EXEC		(1 << 13)
>  #define PERF_RECORD_MISC_FORK_EXEC		(1 << 13)
>  #define PERF_RECORD_MISC_SWITCH_OUT		(1 << 13)
> +
> +#define PERF_RECORD_MISC_KSYMBOL_UNREGISTER	(1 << 3)
> +#define PERF_RECORD_MISC_KSYMBOL_TYPE_MASK	(7 << 4)
> +#define PERF_RECORD_MISC_KSYMBOL_TYPE_UNKNOWN	(0 << 4)
> +#define PERF_RECORD_MISC_KSYMBOL_TYPE_BPF	(1 << 4)

So this gives us 8 possible types, of which 2 are claimed.

I suppose we already know of FTRACE_TRAMPOLINE and MODULE, which
accounts for half the space.

> +
>  /*
>   * These PERF_RECORD_MISC_* flags below are safely reused
>   * for the following events:
> @@ -965,6 +973,19 @@ enum perf_event_type {
>  	 */
>  	PERF_RECORD_NAMESPACES			= 16,
>  
> +	/*
> +	 * Record ksymbol register/unregister events:
> +	 *
> +	 * struct {
> +	 *	struct perf_event_header	header;
> +	 *	u64				addr;
> +	 *	u64				len;
> +	 *	char				name[];
> +	 *	struct sample_id		sample_id;
> +	 * };
> +	 */
> +	PERF_RECORD_KSYMBOL			= 17,

Do we really need u64 length? That is, are we willing to consider
symbols larger than 4G ?

Could we not reclaim the top 32 or 16 bits thereof for that type thing
instead of using a few misc bits?