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Date: Tue, 14 Jul 2015 19:32:02 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
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        Borislav Petkov <bp@suse.de>,
        Yanmin Zhang <yanmin_zhang@linux.intel.com>,
        xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during
 cpu up/down
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On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
> On 07/14/2015 11:44 AM, Thomas Gleixner wrote:
> > On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
> > > > Prevent allocation and freeing of interrupt descriptors accross cpu
> > > > hotplug.
> > > 
> > > This breaks Xen guests that allocate interrupt descriptors in .cpu_up().
> > And where exactly does XEN allocate those descriptors?
> 
> xen_cpu_up()
>     xen_setup_timer()
>         bind_virq_to_irqhandler()
>             bind_virq_to_irq()
>                 xen_allocate_irq_dynamic()
>                     xen_allocate_irqs_dynamic()
>                         irq_alloc_descs()
> 
> 
> There is also a similar pass via xen_cpu_up() -> xen_smp_intr_init()

Sigh.
 
> 
> >   
> > > Any chance this locking can be moved into arch code?
> > No.

The issue here is that all architectures need that protection and just
Xen does irq allocations in cpu_up.

So moving that protection into architecture code is not really an
option.

> > > Otherwise we will need to have something like arch_post_cpu_up()
> > > after the lock is released.

I'm not sure, that this will work. You probably want to do this in the
cpu prepare stage, i.e. before calling __cpu_up().

I have to walk the dogs now. Will look into it later tonight.

> > > (The patch doesn't appear to have any side effects for the down path since
> > > Xen
> > > guests deallocate descriptors in __cpu_die()).
> >   Exact place please.
> 
> Whose place? Where descriptors are deallocated?
> 
> __cpu_die()
>     xen_cpu_die()
>         xen_teardown_timer()
>             unbind_from_irqhandler()
>                 unbind_from_irq()
>                     __unbind_from_irq()
>                         xen_free_irq()
>                             irq_free_descs()
>                                 free_desc()

Right, that's outside the lock held region.

Thanks,

	tglx