linux-kernel - Re: [PATCH] xen/events: xen_evtchn_fifo

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Message-ID: <alpine.DEB.2.02.1401281406090.4373@kaball.uk.xensource.com>
Date:	Tue, 28 Jan 2014 14:30:46 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	Julien Grall <julien.grall@...aro.org>,
	<linux-kernel@...r.kernel.org>, <konrad.wilk@...cle.com>,
	<boris.ostrovsky@...cle.com>, <xen-devel@...ts.xenproject.org>,
	<ian.campbell@...rix.com>, <stefano.stabellini@...citrix.com>,
	<patches@...aro.org>
Subject: Re: [PATCH] xen/events: xen_evtchn_fifo_init can be called very
 late

On Tue, 28 Jan 2014, David Vrabel wrote:
> On 28/01/14 00:34, Julien Grall wrote:
> > On ARM, xen_init_IRQ (which calls xen_evtchn_fifo_init) is called after
> > all CPUs are online. It would mean that the notifier will never be called.
> 
> Why does ARM call xen_init_IRQ() so late?  Is it possible to call it
> earlier when only the boot CPU is online?  There are problems with
> attempting to init FIFO event channels after all CPUs are online.
> 
> If evtchn_fifo_init_control_block(cpu) fails on anything other than the
> first CPU, that CPU will be unable to receive any events.  Xen will have
> been switched to FIFO mode and it is not possible to revert back to
> 2-level mode.

We simply didn't need to be called that early.
Most of xen_guest_init could be moved to an early_initcall, if that is
necessary.



> > Therefore, when a secondary CPU will receive an interrupt, Linux will segfault
> > because the event channel structure for this processor is not initialized.
> > 
> > This can be fixed by calling the init function on every online cpu when the
> > event channel fifo driver is initialized.
> > 
> > Signed-off-by: Julien Grall <julien.grall@...aro.org>
> > ---
> >  drivers/xen/events/events_fifo.c |   11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/xen/events/events_fifo.c b/drivers/xen/events/events_fifo.c
> > index 1de2a19..15498ab 100644
> > --- a/drivers/xen/events/events_fifo.c
> > +++ b/drivers/xen/events/events_fifo.c
> > @@ -410,12 +410,14 @@ static struct notifier_block evtchn_fifo_cpu_notifier = {
> >  
> >  int __init xen_evtchn_fifo_init(void)
> >  {
> > -	int cpu = get_cpu();
> > +	int cpu;
> >  	int ret;
> >  
> > -	ret = evtchn_fifo_init_control_block(cpu);
> > -	if (ret < 0)
> > -		goto out;
> > +	for_each_online_cpu(cpu) {
> > +		ret = evtchn_fifo_init_control_block(cpu);
> > +		if (ret < 0)
> > +			goto out;
> 
> You need to handle this error differently depending on whether the first
> call fails or not.
> 
> Failure on first CPU: return an error and the caller will fallback to
> using 2-level mode.
> 
> Failure on second or later CPU: you need to offline that CPU.  It may
> not be possible to offline a CPU with standard calls (e.g., cpu_down())
> as it won't have working interrupts.
> 
> David
> 
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