[PATCH] powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

[Mahesh Salgaonkar]

nmi_enter()/nmi_exit() touches per cpu variables which can lead to kernel
crash when invoked during real mode interrupt handling (e.g. early HMI/MCE
interrupt handler) if percpu allocation comes from vmalloc area.

Early HMI/MCE handlers are called through DEFINE_INTERRUPT_HANDLER_NMI()
wrapper which invokes nmi_enter/nmi_exit calls. We don't see any issue when
percpu allocation is from the embedded first chunk. However with
CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there are chances where percpu
allocation can come from the vmalloc area.

With kernel command line "percpu_alloc=page" we can force percpu allocation
to come from vmalloc area and can see kernel crash in machine_check_early:

[    1.215714] NIP [c000000000e49eb4] rcu_nmi_enter+0x24/0x110
[    1.215717] LR [c0000000000461a0] machine_check_early+0xf0/0x2c0
[    1.215719] --- interrupt: 200
[    1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable)
[    1.215722] [c000000fffd731b0] [0000000000000000] 0x0
[    1.215724] [c000000fffd73210] [c000000000008364] machine_check_early_common+0x134/0x1f8

Fix this by avoiding use of nmi_enter()/nmi_exit() in real mode if percpu
first chunk is not embedded.

Signed-off-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>
---
 arch/powerpc/include/asm/interrupt.h |   15 +++++++++++++++
 arch/powerpc/include/asm/percpu.h    |    2 ++
 arch/powerpc/kernel/setup_64.c       |    3 +++
 3 files changed, 20 insertions(+)

diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
index fc28f46d2f9dc..8c5b3a7ca2ab6 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h
@@ -327,6 +327,16 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
 	}
 #endif
 
+	/*
+	 * Do not use nmi_enter() in real mode if percpu first chunk is
+	 * not embedded. With CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled
+	 * there are chances where percpu allocation can come from vmalloc
+	 * area.
+	 */
+	if (IS_ENABLED(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) &&
+	    !(mfmsr() & MSR_DR) && !__percpu_embed_first_chunk)
+		return;
+
 	/*
 	 * Do not use nmi_enter() for pseries hash guest taking a real-mode
 	 * NMI because not everything it touches is within the RMA limit.
@@ -339,6 +349,10 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
 
 static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct interrupt_nmi_state *state)
 {
+	if (IS_ENABLED(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) &&
+	    !(mfmsr() & MSR_DR) && !__percpu_embed_first_chunk)
+		goto skip_nmi_exit;
+
 	if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64) ||
 			!firmware_has_feature(FW_FEATURE_LPAR) ||
 			radix_enabled() || (mfmsr() & MSR_DR))
@@ -349,6 +363,7 @@ static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct inter
 	 * new work to do (must use irq_work for that).
 	 */
 
+skip_nmi_exit:
 #ifdef CONFIG_PPC64
 #ifdef CONFIG_PPC_BOOK3S
 	if (arch_irq_disabled_regs(regs)) {
diff --git a/arch/powerpc/include/asm/percpu.h b/arch/powerpc/include/asm/percpu.h
index 8e5b7d0b851c6..45c08f5b0b4e9 100644
--- a/arch/powerpc/include/asm/percpu.h
+++ b/arch/powerpc/include/asm/percpu.h
@@ -12,6 +12,8 @@
 
 #define __my_cpu_offset local_paca->data_offset
 
+extern bool __percpu_embed_first_chunk;
+
 #endif /* CONFIG_SMP */
 #endif /* __powerpc64__ */
 
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index be8577ac93971..39dbf2fb23d61 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -786,6 +786,7 @@ static __init int pcpu_cpu_to_node(int cpu)
 
 unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
 EXPORT_SYMBOL(__per_cpu_offset);
+bool __percpu_embed_first_chunk;
 
 void __init setup_per_cpu_areas(void)
 {
@@ -821,6 +822,8 @@ void __init setup_per_cpu_areas(void)
 			pr_warn("PERCPU: %s allocator failed (%d), "
 				"falling back to page size\n",
 				pcpu_fc_names[pcpu_chosen_fc], rc);
+		else
+			__percpu_embed_first_chunk = true;
 	}
 
 	if (rc < 0)