From mboxrd@z Thu Jan  1 00:00:00 1970
From: mark.rutland@arm.com (Mark Rutland)
Date: Tue, 11 Jun 2013 10:04:59 +0100
Subject: [PATCH] arm: versatile: don't mark pen as __INIT
In-Reply-To: <20130610232441.GI18614@n2100.arm.linux.org.uk>
References: <1370876844-6599-1-git-send-email-mark.rutland@arm.com>
 <20130610232441.GI18614@n2100.arm.linux.org.uk>
Message-ID: <20130611090459.GA9529@e106331-lin.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Tue, Jun 11, 2013 at 12:24:41AM +0100, Russell King - ARM Linux wrote:
> On Mon, Jun 10, 2013 at 04:07:24PM +0100, Mark Rutland wrote:
> > When booting fewer cores than are physically present on a versatile
> > platform (e.g. when passing maxcpus=N on the command line), some
> > secondary cores may remain in the holding pen, which is marked __INIT.
> > Late in the boot process, the memory comprising the holding pen will be
> > released to the kernel for more general use, and may be overwritten with
> > arbitrary data, which can cause the held secondaries to start behaving
> > unpredictably. This can lead to all manner of odd behaviour from the
> > kernel.
> > 
> > Instead don't mark the section as __INIT. This means we can't reuse the
> > pen memory, but we won't get secondaries corrupting the rest of the
> > kernel.
> 
> __CPUINIT is appropriate here; __CPUINIT will be kept around if you have
> hotplug CPU suport, but if you don't it will be discarded after all
> secondary CPUs have booted.  And without hotplug CPU, you can't ask
> for the offline CPUs to be onlined.

Since 384a290283: "ARM: gic: use a private mapping for CPU target interfaces",
each CPU's gic_cpu_map entry is initialised to 0xff, so a call to
gic_raise_softirq will target *all* CPUs attached to the GIC if one of the CPUs
targetted has not been initialised.

Thus any call to versatile_boot_secondary will wake up *all* secondaries
physically present, throwing them all into the pen. If we use a subset of these
(e.g. from having "maxcpus=N" on the command line), some will be left in the
pen, even though we didn't ask for them explicitly. This will happen with or
without CPU_HOTPLUG.

Another option would be to add an optional description of a CPU's gic id to the
dt, which would allow us to avoid throwing these secondaries into the pen in
the first place.

Thanks,
Mark