linux-kernel - Re: [PATCH 22/24] ARM64:ILP32: Use a seperate syscall table as a few syscalls need to be using the compat syscalls.

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Date:	Tue, 1 Jul 2014 17:50:41 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Pinski, Andrew" <Andrew.Pinski@...iumnetworks.com>,
	Andrew Pinski <apinski@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 22/24] ARM64:ILP32: Use a seperate syscall table as a few
 syscalls need to be using the compat syscalls.

On Tue, Jul 01, 2014 at 05:38:12PM +0100, Arnd Bergmann wrote:
> On Tuesday 01 July 2014 15:30:51 Pinski, Andrew wrote:
> > > On Jul 1, 2014, at 8:07 AM, "Catalin Marinas" <catalin.marinas@....com> wrote:
> > >> On Sat, May 24, 2014 at 12:02:17AM -0700, Andrew Pinski wrote:
> > >> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > >> index 1e1ebfc..8241ffe 100644
> > >> --- a/arch/arm64/kernel/entry.S
> > >> +++ b/arch/arm64/kernel/entry.S
> > >> @@ -620,9 +620,14 @@ ENDPROC(ret_from_fork)
> > >>  */
> > >>    .align    6
> > >> el0_svc:
> > >> -    adrp    stbl, sys_call_table        // load syscall table pointer
> > >>    uxtw    scno, w8            // syscall number in w8
> > >>    mov    sc_nr, #__NR_syscalls
> > >> +#ifdef CONFIG_ARM64_ILP32
> > >> +    get_thread_info tsk
> > >> +    ldr    x16, [tsk, #TI_FLAGS]
> > >> +    tbnz    x16, #TIF_32BIT_AARCH64, el0_ilp32_svc // We are using ILP32
> > >> +#endif
> > >> +    adrp    stbl, sys_call_table        // load syscall table pointer
> > > 
> > > This adds a slight penalty on the AArch64 SVC entry path. I can't tell
> > > whether that's visible or not but I think the x86 guys decided to set an
> > > extra bit to the syscall number to distinguish it from native calls.
> 
> IIRC the intention on x86 was that you should always be able to call
> any of the three syscall ABIs (x86-32, x86-64, x32) from any process
> by passing the right number, for flexibility.

I don't see how this is useful though. Do you happen to have more
information?

-- 
Catalin
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