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xen/smp: Fix crash when booting with ACPI hotplug CPUs.
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commit cf405ae upstream.

When we boot on a machine that can hotplug CPUs and we
are using 'dom0_max_vcpus=X' on the Xen hypervisor line
to clip the amount of CPUs available to the initial domain,
we get this:

(XEN) Command line: com1=115200,8n1 dom0_mem=8G noreboot dom0_max_vcpus=8 sync_console mce_verbosity=verbose console=com1,vga loglvl=all guest_loglvl=all
.. snip..
DMI: Intel Corporation S2600CP/S2600CP, BIOS SE5C600.86B.99.99.x032.072520111118 07/25/2011
.. snip.
SMP: Allowing 64 CPUs, 32 hotplug CPUs
installing Xen timer for CPU 7
cpu 7 spinlock event irq 361
NMI watchdog: disabled (cpu7): hardware events not enabled
Brought up 8 CPUs
.. snip..
	[acpi processor finds the CPUs are not initialized and starts calling
	arch_register_cpu, which creates /sys/devices/system/cpu/cpu8/online]
CPU 8 got hotplugged
CPU 9 got hotplugged
CPU 10 got hotplugged
.. snip..
initcall 1_acpi_battery_init_async+0x0/0x1b returned 0 after 406 usecs
calling  erst_init+0x0/0x2bb @ 1

	[and the scheduler sticks newly started tasks on the new CPUs, but
	said CPUs cannot be initialized b/c the hypervisor has limited the
	amount of vCPUS to 8 - as per the dom0_max_vcpus=8 flag.
	The spinlock tries to kick the other CPU, but the structure for that
	is not initialized and we crash.]
BUG: unable to handle kernel paging request at fffffffffffffed8
IP: [<ffffffff81035289>] xen_spin_lock+0x29/0x60
PGD 180d067 PUD 180e067 PMD 0
Oops: 0002 [#1] SMP
CPU 7
Modules linked in:

Pid: 1, comm: swapper/0 Not tainted 3.4.0-rc2upstream-00001-gf5154e8 #1 Intel Corporation S2600CP/S2600CP
RIP: e030:[<ffffffff81035289>]  [<ffffffff81035289>] xen_spin_lock+0x29/0x60
RSP: e02b:ffff8801fb9b3a70  EFLAGS: 00010282

With this patch, we cap the amount of vCPUS that the initial domain
can run, to exactly what dom0_max_vcpus=X has specified.

In the future, if there is a hypercall that will allow a running
domain to expand past its initial set of vCPUS, this patch should
be re-evaluated.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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konradwilk authored and bwhacks committed May 11, 2012
1 parent 585bfaa commit 66cd728
Showing 1 changed file with 15 additions and 0 deletions.
15 changes: 15 additions & 0 deletions arch/x86/xen/smp.c
Original file line number Diff line number Diff line change
Expand Up @@ -172,6 +172,7 @@ static void __init xen_fill_possible_map(void)
static void __init xen_filter_cpu_maps(void)
{
int i, rc;
unsigned int subtract = 0;

if (!xen_initial_domain())
return;
Expand All @@ -186,8 +187,22 @@ static void __init xen_filter_cpu_maps(void)
} else {
set_cpu_possible(i, false);
set_cpu_present(i, false);
subtract++;
}
}
#ifdef CONFIG_HOTPLUG_CPU
/* This is akin to using 'nr_cpus' on the Linux command line.
* Which is OK as when we use 'dom0_max_vcpus=X' we can only
* have up to X, while nr_cpu_ids is greater than X. This
* normally is not a problem, except when CPU hotplugging
* is involved and then there might be more than X CPUs
* in the guest - which will not work as there is no
* hypercall to expand the max number of VCPUs an already
* running guest has. So cap it up to X. */
if (subtract)
nr_cpu_ids = nr_cpu_ids - subtract;
#endif

}

static void __init xen_smp_prepare_boot_cpu(void)
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