From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <jbeulich@novell.com>
Subject: Re: [PATCH] Fix clock_gettime to increment
	monotonically onPV-domain/x86
Date: Fri, 15 Jun 2007 13:12:21 +0200
Message-ID: <46729035.76E4.0078.0@novell.com>
References: <467283C5.76E4.0078.0@novell.com>
	<C29827EB.9300%keir@xensource.com>
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To: Atsushi SAKAI <sakaia@jp.fujitsu.com>, Keir Fraser <keir@xensource.com>
Cc: xen-devel@lists.xensource.com
List-Id: xen-devel@lists.xenproject.org

An alternative to a spin lock would be to use a 64-bit variable storing =
the raw
nanosecond value, and use cmpxchg to check/update it. I did it this way =
for
the clocksource monotonicity:

static cycle_t xen_clocksource_read(void)
{
	int cpu =3D get_cpu();
	struct shadow_time_info *shadow =3D &per_cpu(shadow_time, cpu);
	cycle_t ret;

	get_time_values_from_xen(cpu);

	ret =3D shadow->system_timestamp + get_nsec_offset(shadow);

	put_cpu();

#ifdef CONFIG_SMP
	for (;;) {
		static cycle_t last_ret;
#ifndef CONFIG_64BIT
		cycle_t last =3D cmpxchg64(&last_ret, 0, 0);
#else
		cycle_t last =3D last_ret;
#define cmpxchg64 cmpxchg
#endif

		if ((s64)(ret - last) < 0) {
			if (last - ret > permitted_clock_jitter
			    && printk_ratelimit())
				printk(KERN_WARNING "clocksource/%d: "
				       "Time went backwards: "
				       "delta=3D%Ld shadow=3D%Lu offset=3D%=
Lu\n",
				       cpu, ret - last,
				       shadow->system_timestamp,
				       get_nsec_offset(shadow));
			ret =3D last;
		}
		if (cmpxchg64(&last_ret, last, ret) =3D=3D last)
			break;
	}
#endif

	return ret;
}

Jan

>>> Keir Fraser <keir@xensource.com> 15.06.07 12:26 >>>
Yeah, it needs a spinlock. Which is a shame, but it's still going to be =
much
faster than trapping into the hypervisor, which is the only other way =
we're
going to be able to achieve guaranteed monotonic time.

We could avoid the lock if we guaranteed monotonic time only per task
(thread). Or is that not really good enough? :-)

 -- Keir

On 15/6/07 11:19, "Jan Beulich" <jbeulich@novell.com> wrote:

> I'm afraid this isn't MP-safe - you're in a (pseudo-)read-locked section =
of
> code,
> yet write a global variable (and even in two pieces). Jan
>=20
>>>> Atsushi SAKAI <sakaia@jp.fujitsu.com> 15.06.07 11:47 >>>
> Hi, Keir
>=20
>   This patch intends to increment do_gettimeofday monotonically.
> This is for PV-domain/x86.
>=20
> Signed-off-by: Atsushi SAKAI <sakaia@jp.fujitsu.com>
>=20
> By applying patch, the time rollback for SMP is fixed.
> This is important fixes for database software.
>=20
> Thanks
> Atsushi SAKAI
>=20
>=20
>=20
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com=20
> http://lists.xensource.com/xen-devel=20