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From: Linus Walleij <linus.walleij@linaro.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@lst.de>,
	Russell King <rmk@arm.linux.org.uk>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"linux-kernel@vger.kernel.org" <kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Russell King <linux@armlinux.org.uk>,
	Oleg Nesterov <oleg@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/9] ARM: syscall: always store thread_info->syscall
Date: Mon, 28 Sep 2020 11:41:24 +0200	[thread overview]
Message-ID: <CACRpkdYkL2=gkBvbHO514rnppLdHgsXwi0==6Ovq43kSZqEvUQ@mail.gmail.com> (raw)
In-Reply-To: <20200907153701.2981205-5-arnd@arndb.de>

Hi Arnd,

help me out here because I feel vaguely stupid...

On Mon, Sep 7, 2020 at 5:38 PM Arnd Bergmann <arnd@arndb.de> wrote:

>  {
> +       if (IS_ENABLED(CONFIG_OABI_COMPAT))
> +               return task_thread_info(task)->syscall & ~__NR_OABI_SYSCALL_BASE;

Where __NR_OABI_SYSCALL_BASE is
#define __NR_OABI_SYSCALL_BASE       0x900000

So you will end up with sycall number & FF6FFFFF
masking off bits 20 and 23.

I suppose this is based on this:

>         bics    r10, r10, #0xff000000
> +       str     r10, [tsk, #TI_SYSCALL]

OK we mask off bits 24-31 before we store this.

>         bic     scno, scno, #0xff000000         @ mask off SWI op-code
> +       str     scno, [tsk, #TI_SYSCALL]

And here too.

>         eor     scno, scno, #__NR_SYSCALL_BASE  @ check OS number

And then happens that which will ... I don't know really.
Exclusive or with 0x9000000 is not immediately intuitive
evident to me, I suppose it is for everyone else... :/

I need some idea how this numberspace is managed in order to
understand the code so I can review it, I guess it all makes perfect
sense but I need some background here.

Thanks,
Linus Walleij

  reply	other threads:[~2020-09-28  9:41 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-07 15:36 [PATCH 0/9] ARM: remove set_fs callers and implementation Arnd Bergmann
2020-09-07 15:36 ` [PATCH 1/9] mm/maccess: fix unaligned copy_{from,to}_kernel_nofault Arnd Bergmann
2020-09-08  6:11   ` Christoph Hellwig
2020-09-27  9:25   ` Linus Walleij
2020-09-07 15:36 ` [PATCH 2/9] ARM: traps: use get_kernel_nofault instead of set_fs() Arnd Bergmann
2020-09-08  6:15   ` Christoph Hellwig
2020-09-17 17:29     ` Arnd Bergmann
2020-09-18  7:42       ` Russell King - ARM Linux admin
2020-09-18 12:36         ` Arnd Bergmann
2020-09-07 15:36 ` [PATCH 3/9] ARM: oabi-compat: add epoll_pwait handler Arnd Bergmann
2020-09-08  6:16   ` Christoph Hellwig
2020-09-07 15:36 ` [PATCH 4/9] ARM: syscall: always store thread_info->syscall Arnd Bergmann
2020-09-28  9:41   ` Linus Walleij [this message]
2020-09-28 12:42     ` Arnd Bergmann
2020-09-28 15:08       ` Russell King - ARM Linux admin
2020-09-07 15:36 ` [PATCH 5/9] ARM: oabi-compat: rework epoll_wait/epoll_pwait emulation Arnd Bergmann
2020-09-08  6:20   ` Christoph Hellwig
2020-09-08 20:56     ` Arnd Bergmann
2020-09-07 15:36 ` [PATCH 6/9] ARM: oabi-compat: rework sys_semtimedop emulation Arnd Bergmann
2020-09-07 15:36 ` [PATCH 7/9] ARM: oabi-compat: rework fcntl64() emulation Arnd Bergmann
2020-09-07 15:36 ` [PATCH 8/9] ARM: uaccess: add __{get,put}_kernel_nofault Arnd Bergmann
2020-09-07 15:36 ` [PATCH 9/9] ARM: uaccess: remove set_fs() implementation Arnd Bergmann
  -- strict thread matches above, loose matches on Subject: below --
2020-10-30 15:45 [PATCH v4 0/9] ARM: remove set_fs callers and implementation Arnd Bergmann
2020-10-30 15:49 ` [PATCH 1/9] mm/maccess: fix unaligned copy_{from,to}_kernel_nofault Arnd Bergmann
2020-10-30 15:49   ` [PATCH 4/9] ARM: syscall: always store thread_info->syscall Arnd Bergmann
2020-10-30 16:53     ` Russell King - ARM Linux admin
2020-10-30 21:28       ` Arnd Bergmann

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