linux-kernel - Re: [PATCH v2 3/7] perf syscalltbl: Remove struct syscalltbl

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Message-Id: <07c5c3ad-5a6d-4eda-95f2-ed16e7504d4c@app.fastmail.com>
Date: Tue, 11 Feb 2025 08:48:26 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ian Rogers" <irogers@...gle.com>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Ingo Molnar" <mingo@...hat.com>,
 "Arnaldo Carvalho de Melo" <acme@...nel.org>,
 "Namhyung Kim" <namhyung@...nel.org>,
 "Mark Rutland" <mark.rutland@....com>,
 "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
 "Jiri Olsa" <jolsa@...nel.org>,
 "Adrian Hunter" <adrian.hunter@...el.com>,
 "Kan Liang" <kan.liang@...ux.intel.com>,
 "John Garry" <john.g.garry@...cle.com>, "Will Deacon" <will@...nel.org>,
 "James Clark" <james.clark@...aro.org>,
 "Mike Leach" <mike.leach@...aro.org>, "Leo Yan" <leo.yan@...ux.dev>,
 guoren <guoren@...nel.org>, "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>,
 "Albert Ou" <aou@...s.berkeley.edu>,
 "Charlie Jenkins" <charlie@...osinc.com>,
 "Bibo Mao" <maobibo@...ngson.cn>, "Huacai Chen" <chenhuacai@...nel.org>,
 "Catalin Marinas" <catalin.marinas@....com>,
 "Jiri Slaby" <jirislaby@...nel.org>,
 Björn Töpel <bjorn@...osinc.com>,
 "Howard Chu" <howardchu95@...il.com>, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
 linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 3/7] perf syscalltbl: Remove struct syscalltbl

On Mon, Feb 10, 2025, at 17:51, Ian Rogers wrote:
> The syscalltbl held entries of system call name and number pairs,
> generated from a native syscalltbl at start up. As there are gaps in
> the system call number there is a notion of index into the
> table. Going forward we want the system call table to be identifiable
> by a machine type, for example, i386 vs x86-64. Change the interface
> to the syscalltbl so (1) a (currently unused machine type of EM_HOST)
> is passed (2) the index to syscall number and system call name mapping
> is computed at build time.
>
> Two tables are used for this, an array of system call number to name,
> an array of system call numbers sorted by the system call name. The
> sorted array doesn't store strings in part to save memory and
> relocations. The index notion is carried forward and is an index into
> the sorted array of system call numbers, the data structures are
> opaque (held only in syscalltbl.c), and so the number of indices for a
> machine type is exposed as a new API.
>
> The arrays are computed in the syscalltbl.sh script and so no start-up
> time computation and storage is necessary.
>
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> Reviewed-by: Howard Chu <howardchu95@...il.com>

Your changes look fine to me, but I noticed one part that may
be wrong before and after your patch:

> 
> -const int syscalltbl_native_max_id = SYSCALLTBL_MAX_ID;
> -static const char *const *syscalltbl_native = syscalltbl;
> +const char *syscalltbl__name(int e_machine __maybe_unused, int id)
> +{
> +	if (id >= 0 && id <= (int)ARRAY_SIZE(syscall_num_to_name))
> +		return syscall_num_to_name[id];
> +	return NULL;
> +}

The syscall numbers on mips (and previously on ia64) are offset by
a large number depending on the ABI (o32/n32/n64). I assume what
we want here is to have the small numbers without the offset in
syscall_num_to_name[], but that requires adding the offset during
the lookup. Can you check if this is handled correctly?

      Arnd