linux-kernel - Re: [PATCH v2 2/4] MIPS: VDSO: Add implementation of clock

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Date:   Thu, 6 Jul 2017 10:05:53 +0100
From:   James Hogan <james.hogan@...tec.com>
To:     "Maciej W. Rozycki" <macro@...tec.com>
CC:     Aleksandar Markovic <aleksandar.markovic@...rk.com>,
        <linux-mips@...ux-mips.org>,
        Goran Ferenc <goran.ferenc@...tec.com>,
        Miodrag Dinic <miodrag.dinic@...tec.com>,
        Aleksandar Markovic <aleksandar.markovic@...tec.com>,
        Douglas Leung <douglas.leung@...tec.com>,
        <linux-kernel@...r.kernel.org>,
        Paul Burton <paul.burton@...tec.com>,
        Petar Jovanovic <petar.jovanovic@...tec.com>,
        Raghu Gandham <raghu.gandham@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH v2 2/4] MIPS: VDSO: Add implementation of clock_gettime()
 fallback

On Thu, Jul 06, 2017 at 01:00:34AM +0100, Maciej W. Rozycki wrote:
> On Wed, 28 Jun 2017, Aleksandar Markovic wrote:
> 
> > diff --git a/arch/mips/vdso/gettimeofday.c b/arch/mips/vdso/gettimeofday.c
> > index fd7d433..5f63375 100644
> > --- a/arch/mips/vdso/gettimeofday.c
> > +++ b/arch/mips/vdso/gettimeofday.c
> > @@ -20,6 +20,24 @@
> >  #include <asm/unistd.h>
> >  #include <asm/vdso.h>
> >  
> > +static __always_inline long clock_gettime_fallback(clockid_t _clkid,
> > +					   struct timespec *_ts)
> > +{
> > +	register struct timespec *ts asm("a1") = _ts;
> > +	register clockid_t clkid asm("a0") = _clkid;
> > +	register long ret asm("v0");
> > +	register long nr asm("v0") = __NR_clock_gettime;
> > +	register long error asm("a3");
> > +
> > +	asm volatile(
> > +	"       syscall\n"
> > +	: "=r" (ret), "=r" (error)
> > +	: "r" (clkid), "r" (ts), "r" (nr)
> > +	: "memory");
> > +
> > +	return error ? -ret : ret;
> > +}
> 
>  Hmm, are you sure it is safe nowadays WRT the syscall restart convention 
> to leave out the instruction explicitly loading the syscall number that 
> would normally immediately precede SYSCALL

It should be fine. syscall restart only rewinds one (32-bit)
instruction, and it preserves the syscall number in pt_regs::regs[0]
(see handle_signal() / do_signal() and this code in e.g. scall32-o32.S:)

sw      t1, PT_R0(sp)           # save it for syscall restarting

> (and would have to forcibly use the 32-bit encoding in the microMIPS
> case)?

I don't believe there is a 16-bit SYSCALL encoding in microMIPS, at
least I can't see one in the 5.04 manual.

However, the clobber list is incomplete.
The following registers are written as outputs:
	$2 (v0), $7 (a3)
The following registers are used as arguments and should be preserved:
	$4-$6 (a0-a2), [$8-$9 (a4-a5)] (n32 / n64 only)
And the following other registers are preserved:
	$16-$23, $28-$31
So assuming you already have $2 and $7 as outputs, the clobber list
should be:
	"$1", "$3", ["$8", "$9",] "$10", "$11", "$12", "$13", "$14",
	"$15", "$24", "$25", "hi", "lo", "memory"

(only o32 needs to mark $8-$9 clobbered, but no harm doing so on n32/n64
too)

Cheers
James

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