xen: mask x2APIC feature in PV

Zhenzhong Duan · konradwilk · commit 4ea9b9aca90c · 2013-05-08T08:38:11.000-04:00
On x2apic enabled pvm, doing sysrq+l, got NULL pointer dereference as below.

    SysRq : Show backtrace of all active CPUs
    BUG: unable to handle kernel NULL pointer dereference at           (null)
    IP: [&lt;ffffffff8125e3cb&gt;] memcpy+0xb/0x120
    Call Trace:
     [&lt;ffffffff81039633&gt;] ? __x2apic_send_IPI_mask+0x73/0x160
     [&lt;ffffffff8103973e&gt;] x2apic_send_IPI_all+0x1e/0x20
     [&lt;ffffffff8103498c&gt;] arch_trigger_all_cpu_backtrace+0x6c/0xb0
     [&lt;ffffffff81501be4&gt;] ? _raw_spin_lock_irqsave+0x34/0x50
     [&lt;ffffffff8131654e&gt;] sysrq_handle_showallcpus+0xe/0x10
     [&lt;ffffffff8131616d&gt;] __handle_sysrq+0x7d/0x140
     [&lt;ffffffff81316230&gt;] ? __handle_sysrq+0x140/0x140
     [&lt;ffffffff81316287&gt;] write_sysrq_trigger+0x57/0x60
     [&lt;ffffffff811ca996&gt;] proc_reg_write+0x86/0xc0
     [&lt;ffffffff8116dd8e&gt;] vfs_write+0xce/0x190
     [&lt;ffffffff8116e3e5&gt;] sys_write+0x55/0x90
     [&lt;ffffffff8150a242&gt;] system_call_fastpath+0x16/0x1b

That's because apic points to apic_x2apic_cluster or apic_x2apic_phys
but the basic element like cpumask isn't initialized.

Mask x2APIC feature in pvm to avoid overwrite of apic pointer,
update commit message per Konrad's suggestion.

Signed-off-by: Zhenzhong Duan &lt;zhenzhong.duan@oracle.com&gt;
Tested-by: Tamon Shiose &lt;tamon.shiose@oracle.com&gt;
Signed-off-by: Konrad Rzeszutek Wilk &lt;konrad.wilk@oracle.com&gt;
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
@@ -429,6 +429,9 @@ static void __init xen_init_cpuid_mask(void)
 		cpuid_leaf1_edx_mask &=
 			~((1 << X86_FEATURE_APIC) |  /* disable local APIC */
 			  (1 << X86_FEATURE_ACPI));  /* disable ACPI */
+
+	cpuid_leaf1_ecx_mask &= ~(1 << (X86_FEATURE_X2APIC % 32));
+
 	ax = 1;
 	cx = 0;
 	xen_cpuid(&ax, &bx, &cx, &dx);
