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Subject[PATCH-v2] xen: Don't call arch_trigger_all_cpu_backtrace in dom0(pvm)
nmi isn't supported in dom0, fallback to general all cpu backtrace code.

Without fix, on xAPIC system, doing sysrq+l, no backtrace is showed,
as xen_send_IPI_all is called and it doesn't support nmi vector.

On x2APIC enabled system, got NULL pointer dereference as below.

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

-v2: Mask x2APIC feature in pvm to avoid overwrite of apic pointer,
update commit message per Konrad's suggestion.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Tested-by: Tamon Shiose <tamon.shiose@oracle.com>
---
arch/x86/xen/enlighten.c | 3 +++
include/linux/nmi.h | 2 ++
2 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index c8e1c7b..12b0718 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -386,6 +386,9 @@ 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);
diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index db50840..b845757 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -32,6 +32,8 @@ static inline void touch_nmi_watchdog(void)
#ifdef arch_trigger_all_cpu_backtrace
static inline bool trigger_all_cpu_backtrace(void)
{
+ if (xen_domain())
+ return false;
arch_trigger_all_cpu_backtrace();

return true;
--
1.7.3


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