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

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Message-ID: <CAP-5=fWjJbg_q3xN0iytcqdBzSuRX+PD7CKp9+rHvFxB1VYxhw@mail.gmail.com>
Date: Tue, 11 Feb 2025 09:32:30 -0800
From: Ian Rogers <irogers@...gle.com>
To: Arnd Bergmann <arnd@...db.de>
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 8:34 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> 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'.

Thanks for clarifying. I believe it will use 6002 and be broken. I
believe that'd be true without these changes too. I'm not testing on
MIPS so it'd be nice to have a fix targeted at making it work. Ideally
we wouldn't use __NR_Linux and instead fudge the id/system call number
based on e_machine/e_flags. I think it is follow on work, especially
because I don't find MIPS a very motivating use-case.

Thanks,
Ian