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[Xen-devel] [PATCH 3/4] xen/smp: Fix crash when booting with ACPI hotplug CPUs.

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
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@xxxxxxxxxx>
 arch/x86/xen/smp.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 5fac691..05da787 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -178,6 +178,7 @@ static void __init xen_fill_possible_map(void)
 static void __init xen_filter_cpu_maps(void)
        int i, rc;
+       unsigned int subtract;
        if (!xen_initial_domain())
@@ -192,8 +193,22 @@ static void __init xen_filter_cpu_maps(void)
                } else {
                        set_cpu_possible(i, false);
                        set_cpu_present(i, false);
+                       subtract++;
+       /* 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;
 static void __init xen_smp_prepare_boot_cpu(void)

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