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

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Message-Id: <8ed7dfb2-1e4d-4aa4-a04b-0397a89365d1@app.fastmail.com>
Date: Tue, 11 Feb 2025 17:34:14 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ian Rogers" <irogers@...gle.com>
Cc: "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 Tue, Feb 11, 2025, at 17:18, Ian Rogers wrote:
> On Mon, Feb 10, 2025 at 11:48 PM Arnd Bergmann <arnd@...db.de> wrote:

>> 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?
>
> Thanks Arnd! I agree the tables are large and can be sparse, they'll
> also be full of relocations. MIPS doesn't look like an outlier to me
> here:

Sorry, I should have been clearer what I meant, see
arch/mips/include/uapi/asm/unistd.h:

#if _MIPS_SIM == _MIPS_SIM_NABI32
#define __NR_Linux      6000
#include <asm/unistd_n32.h>
#endif

and
arch/mips/include/generated/uapi/asm/unistd_n32.h

#define __NR_read (__NR_Linux + 0)
#define __NR_write (__NR_Linux + 1)
#define __NR_open (__NR_Linux + 2)

These offsets are 4000/5000/6000 respectively.

> ```
> #if defined(ALL_SYSCALLTBL) || defined(__mips__)
> static const char *const syscall_num_to_name_EM_MIPS[] = {
>        [0] = "read",
>        [1] = "write",
>        [2] = "open",
> ...
>         [465] = "listxattrat",
>         [466] = "removexattrat",
> };

This means the array is not sparse, but the numbers
here do not match the syscall number argument register.

The question is whether tracing on mips adds the same
per-ABI offset again, or if it tries and fails to look
up index 6002 for 'open'.

       Arnd