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[Xen-devel] [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@xxxxxxxxxx>
Tested-by: Tamon Shiose <tamon.shiose@xxxxxxxxxx>
 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;
        return true;

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