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From:   Alistair Francis <alistair23@gmail.com>
Date:   Fri, 24 Sep 2021 14:34:56 +1000
Message-ID: <CAKmqyKM3vy_Ob1a0=cEi_Efi6_VVAa8YfMb6+rOt3fQK_TTcmw@mail.gmail.com>
Subject: Re: [PATCH v3 2/2] perf bench: Add support for 32-bit systems with
 64-bit time_t
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     =?UTF-8?Q?Andr=C3=A9_Almeida?= <andrealmeid@collabora.com>,
        Alistair Francis <alistair.francis@opensource.wdc.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, linux-perf-users@vger.kernel.org,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Darren Hart <dvhart@infradead.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Atish Patra <atish.patra@wdc.com>,
        Alistair Francis <alistair.francis@wdc.com>
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On Tue, Sep 21, 2021 at 6:08 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Tue, Sep 21, 2021 at 12:47 AM Andr=C3=A9 Almeida
> <andrealmeid@collabora.com> wrote:
> >
> > #if defined(__i386__) || __TIMESIZE =3D=3D 32
> > # define NR_gettime64 __NR_clock_gettime64
> > #else
> > # define NR_gettime64 __NR_clock_gettime
> > #endif
> >
> > struct timespec64 {
> >         long long tv_sec;       /* seconds */
> >         long long tv_nsec;      /* nanoseconds */
> > };
> >
> > int gettime64(clock_t clockid, struct timespec64 *tv)
> > {
> >         return syscall(NR_gettime64, clockid, tv);
> > }
> >
> > Then we can just use &timeout at __NR_futex_time64 for 32bit arch and a=
t
> > __NR_futex for 64bit arch.
>
> This is still broken when you disable CONFIG_COMPAT_32BIT_TIME,
> which disables all system calls that take time32 arguments.
>
> > This might be a simpler solution to the problem that you are facing but
> > I'm not entirely sure. Also, futex's selftests do use the timeout
> > argument and I think that they also won't compile in 32-bit RISC-V, so
> > maybe we can start from there so we can actually test the timeout
> > argument and check if it's working.
>
> I would love to see the wrapper that Alistair wrote as part of some kerne=
l
> uapi header provided to user space. futex is used by tons of applications=
,
> and we never had a library abstraction for it, so everyone has to do thes=
e
> by hand, and they all get them slightly wrong in different ways.
>
> We normally don't do this in kernel headers, but I think the benefits
> would be far greater compared to today's situation.

I'm happy to prepare a patch, if others are on board with it being accepted=
.

Alistair

>
>       Arnd