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Re: [Xen-devel] [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during cpu up/down

On 07/14/2015 04:15 PM, Thomas Gleixner wrote:
On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
On 07/14/2015 01:32 PM, Thomas Gleixner wrote:
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
This breaks Xen guests that allocate interrupt descriptors in
And where exactly does XEN allocate those descriptors?

There is also a similar pass via xen_cpu_up() -> xen_smp_intr_init()
Any chance this locking can be moved into arch code?
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

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().
For PV guests (the ones that use xen_cpu_up()) it will work either before or
after __cpu_up(). At least my (somewhat limited) testing didn't show any
problems so far.

However, HVM CPUs use xen_hvm_cpu_up() and if you read comments there you will
see that xen_smp_intr_init() needs to be called before native_cpu_up() but
xen_init_lock_cpu() (which eventually calls irq_alloc_descs()) needs to be
called after.

I think I can split xen_init_lock_cpu() so that the part that needs to be
called after will avoid going into irq core code. And then the rest will go
into arch_cpu_prepare().
I think we should revisit this for 4.3. For 4.2 we can do the trivial
variant and move the locking in native_cpu_up() and x86 only. x86 was
the only arch on which such wreckage has been seen in the wild, but we
should have that protection for all archs in the long run.

Patch below should fix the issue.

Thanks! Most of my tests passed, I had a couple of failures but I will need to see whether they are related to this patch.


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