[RFC PATCH v3 7/7] arm64: Implement IPI_CPU_BACKTRACE using pseudo-NMIs

From: Daniel Thompson
Date: Fri Aug 19 2016 - 12:13:49 EST

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Recently arm64 gained the capability to (optionally) mask interrupts
using the GIC PMR rather than the CPU PSR. That allows us to introduce
an NMI-like means to handle backtrace requests.

This provides a useful debug aid by allowing the kernel to robustly show
a backtrace for every processor in the system when, for example, we hang
trying to acquire a spin lock.

Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/assembler.h | 23 ++++++++++
arch/arm64/include/asm/smp.h | 2 +
arch/arm64/kernel/entry.S | 78 +++++++++++++++++++++++++++-------
arch/arm64/kernel/smp.c | 27 +++++++++++-
drivers/irqchip/irq-gic-v3.c | 62 +++++++++++++++++++++++++++
include/linux/irqchip/arm-gic-common.h | 8 ++++
include/linux/irqchip/arm-gic.h | 5 ---
8 files changed, 185 insertions(+), 21 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 56846724d2af..7421941b1036 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -86,6 +86,7 @@ config ARM64
select HAVE_IRQ_TIME_ACCOUNTING
select HAVE_MEMBLOCK
select HAVE_MEMBLOCK_NODE_MAP if NUMA
+ select HAVE_NMI
select HAVE_PATA_PLATFORM
select HAVE_PERF_EVENTS
select HAVE_PERF_REGS
diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 0adea5807ef0..840f2f01c60a 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -33,6 +33,29 @@
#include <asm/thread_info.h>

/*
+ * Enable and disable pseudo NMI.
+ */
+ .macro disable_nmi
+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
+ nop
+alternative_else
+ msr daifset, #2
+alternative_endif
+#endif
+ .endm
+
+ .macro enable_nmi
+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
+ nop
+alternative_else
+ msr daifclr, #2
+alternative_endif
+#endif
+ .endm
+
+/*
* Enable and disable interrupts.
*/
.macro disable_irq, tmp
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index 022644704a93..75e0be7ad306 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -33,6 +33,8 @@
#include <linux/cpumask.h>
#include <linux/thread_info.h>

+#define SMP_IPI_NMI_MASK (1 << 6)
+
#define raw_smp_processor_id() (current_thread_info()->cpu)

struct seq_file;
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 1712b344cbf3..8f51e23cb05d 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -268,6 +268,40 @@ alternative_endif
mov sp, x19
.endm

+ .macro trace_hardirqs_off, pstate
+#ifdef CONFIG_TRACE_IRQFLAGS
+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
+ bl trace_hardirqs_off
+ nop
+alternative_else
+ tbnz \pstate, #PSR_G_SHIFT, 1f // PSR_G_BIT
+ bl trace_hardirqs_off
+1:
+alternative_endif
+#else
+ bl trace_hardirqs_off
+#endif
+#endif
+ .endm
+
+ .macro trace_hardirqs_on, pstate
+#ifdef CONFIG_TRACE_IRQFLAGS
+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
+ bl trace_hardirqs_on
+ nop
+alternative_else
+ tbnz \pstate, #PSR_G_SHIFT, 1f // PSR_G_BIT
+ bl trace_hardirqs_on
+1:
+alternative_endif
+#else
+ bl trace_hardirqs_on
+#endif
+#endif
+ .endm
+
/*
* These are the registers used in the syscall handler, and allow us to
* have in theory up to 7 arguments to a function - x0 to x6.
@@ -413,20 +447,19 @@ el1_da:
* Data abort handling
*/
mrs x0, far_el1
+ enable_nmi
enable_dbg
// re-enable interrupts if they were enabled in the aborted context
#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
tbnz x23, #7, 1f // PSR_I_BIT
nop
- nop
msr daifclr, #2
1:
alternative_else
tbnz x23, #PSR_G_SHIFT, 1f // PSR_G_BIT
mov x2, #ICC_PMR_EL1_UNMASKED
msr_s ICC_PMR_EL1, x2
- msr daifclr, #2
1:
alternative_endif
#else
@@ -439,6 +472,7 @@ alternative_endif

// disable interrupts before pulling preserved data off the stack
disable_irq x21
+ disable_nmi
kernel_exit 1
el1_sp_pc:
/*
@@ -479,10 +513,14 @@ ENDPROC(el1_sync)
el1_irq:
kernel_entry 1
enable_dbg
-#ifdef CONFIG_TRACE_IRQFLAGS
- bl trace_hardirqs_off
-#endif
+ trace_hardirqs_off x23

+ /*
+ * On systems with CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS then
+ * we do not yet know if this IRQ is a pseudo-NMI or a normal
+ * interrupt. For that reason we must rely on the irq_handler to
+ * enable the NMI once the interrupt type is determined.
+ */
irq_handler

#ifdef CONFIG_PREEMPT
@@ -493,9 +531,9 @@ el1_irq:
bl el1_preempt
1:
#endif
-#ifdef CONFIG_TRACE_IRQFLAGS
- bl trace_hardirqs_on
-#endif
+
+ disable_nmi
+ trace_hardirqs_on x23
kernel_exit 1
ENDPROC(el1_irq)

@@ -592,6 +630,7 @@ el0_da:
*/
mrs x26, far_el1
// enable interrupts before calling the main handler
+ enable_nmi
enable_dbg_and_irq x0
ct_user_exit
bic x0, x26, #(0xff << 56)
@@ -605,6 +644,7 @@ el0_ia:
*/
mrs x26, far_el1
// enable interrupts before calling the main handler
+ enable_nmi
enable_dbg_and_irq x0
ct_user_exit
mov x0, x26
@@ -638,6 +678,7 @@ el0_sp_pc:
*/
mrs x26, far_el1
// enable interrupts before calling the main handler
+ enable_nmi
enable_dbg_and_irq x0
ct_user_exit
mov x0, x26
@@ -650,6 +691,7 @@ el0_undef:
* Undefined instruction
*/
// enable interrupts before calling the main handler
+ enable_nmi
enable_dbg_and_irq x0
ct_user_exit
mov x0, sp
@@ -692,16 +734,18 @@ el0_irq:
kernel_entry 0
el0_irq_naked:
enable_dbg
-#ifdef CONFIG_TRACE_IRQFLAGS
- bl trace_hardirqs_off
-#endif
-
+ trace_hardirqs_off x23
ct_user_exit
+
+ /*
+ * On systems with CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS then
+ * we do not yet know if this IRQ is a pseudo-NMI or a normal
+ * interrupt. For that reason we must rely on the irq_handler to
+ * enable the NMI once the interrupt type is determined.
+ */
irq_handler

-#ifdef CONFIG_TRACE_IRQFLAGS
- bl trace_hardirqs_on
-#endif
+ trace_hardirqs_on x23
b ret_to_user
ENDPROC(el0_irq)

@@ -751,6 +795,7 @@ ret_fast_syscall:
and x2, x1, #_TIF_WORK_MASK
cbnz x2, work_pending
enable_step_tsk x1, x2
+ disable_nmi
kernel_exit 0
ret_fast_syscall_trace:
enable_irq x0 // enable interrupts
@@ -763,6 +808,7 @@ work_pending:
tbnz x1, #TIF_NEED_RESCHED, work_resched
/* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
mov x0, sp // 'regs'
+ enable_nmi
enable_irq x21 // enable interrupts for do_notify_resume()
bl do_notify_resume
b ret_to_user
@@ -781,6 +827,7 @@ ret_to_user:
and x2, x1, #_TIF_WORK_MASK
cbnz x2, work_pending
enable_step_tsk x1, x2
+ disable_nmi
kernel_exit 0
ENDPROC(ret_to_user)

@@ -806,6 +853,7 @@ el0_svc:
mov sc_nr, #__NR_syscalls
el0_svc_naked: // compat entry point
stp x0, scno, [sp, #S_ORIG_X0] // save the original x0 and syscall number
+ enable_nmi
enable_dbg_and_irq x16
ct_user_exit 1

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 2c118b2db7b4..309e988b00a3 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -879,6 +879,13 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
#endif

case IPI_CPU_BACKTRACE:
+ BUILD_BUG_ON(SMP_IPI_NMI_MASK != BIT(IPI_CPU_BACKTRACE));
+
+ if (in_nmi()) {
+ nmi_cpu_backtrace(regs);
+ break;
+ }
+
printk_nmi_enter();
irq_enter();
nmi_cpu_backtrace(regs);
@@ -960,13 +967,31 @@ bool cpus_are_stuck_in_kernel(void)
return !!cpus_stuck_in_kernel || smp_spin_tables;
}

+/*
+ * IPI_CPU_BACKTRACE is either implemented either as a normal IRQ or,
+ * if the hardware can supports it, using a pseudo-NMI.
+ *
+ * The mechanism used to implement pseudo-NMI means that in both cases
+ * testing if the backtrace IPI is disabled requires us to check the
+ * PSR I bit. However in the later case we cannot use irqs_disabled()
+ * to check the I bit because, when the pseudo-NMI is active that
+ * function examines the GIC PMR instead.
+ */
+static unsigned long nmi_disabled(void)
+{
+ unsigned long flags;
+
+ asm volatile("mrs %0, daif" : "=r"(flags) :: "memory");
+ return flags & PSR_I_BIT;
+}
+
static void raise_nmi(cpumask_t *mask)
{
/*
* Generate the backtrace directly if we are running in a
* calling context that is not preemptible by the backtrace IPI.
*/
- if (cpumask_test_cpu(smp_processor_id(), mask) && irqs_disabled())
+ if (cpumask_test_cpu(smp_processor_id(), mask) && nmi_disabled())
nmi_cpu_backtrace(NULL);

smp_cross_call(mask, IPI_CPU_BACKTRACE);
diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index 14b2fb5e81ef..2b01bfdf0716 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -23,6 +23,7 @@
#include <linux/delay.h>
#include <linux/interrupt.h>
#include <linux/irqdomain.h>
+#include <linux/nmi.h>
#include <linux/of.h>
#include <linux/of_address.h>
#include <linux/of_irq.h>
@@ -342,10 +343,60 @@ static u64 gic_mpidr_to_affinity(unsigned long mpidr)
return aff;
}

+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+static bool gic_handle_nmi(struct pt_regs *regs)
+{
+ u64 irqnr;
+ struct pt_regs *old_regs;
+
+ asm volatile("mrs_s %0, " __stringify(ICC_IAR1_EL1) : "=r"(irqnr));
+
+ /*
+ * If no IRQ is acknowledged at this point then we have entered the
+ * handler due to an normal interrupt (rather than a pseudo-NMI).
+ * If so then unmask the I-bit and return to normal handling.
+ */
+ if (irqnr == ICC_IAR1_EL1_SPURIOUS) {
+ asm volatile("msr daifclr, #2" : : : "memory");
+ return false;
+ }
+
+ old_regs = set_irq_regs(regs);
+ nmi_enter();
+
+ do {
+ if (SMP_IPI_NMI_MASK & (1 << irqnr)) {
+ gic_write_eoir(irqnr);
+ if (static_key_true(&supports_deactivate))
+ gic_write_dir(irqnr);
+ nmi_cpu_backtrace(regs);
+ } else if (unlikely(irqnr != ICC_IAR1_EL1_SPURIOUS)) {
+ gic_write_eoir(irqnr);
+ if (static_key_true(&supports_deactivate))
+ gic_write_dir(irqnr);
+ WARN_ONCE(true, "Unexpected NMI received!\n");
+ }
+
+ asm volatile("mrs_s %0, " __stringify(ICC_IAR1_EL1)
+ : "=r"(irqnr));
+ } while (irqnr != ICC_IAR1_EL1_SPURIOUS);
+
+ nmi_exit();
+ set_irq_regs(old_regs);
+
+ return true;
+}
+#else
+static bool gic_handle_nmi(struct pt_regs *regs) { return false; }
+#endif
+
static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
{
u32 irqnr;

+ if (gic_handle_nmi(regs))
+ return;
+
do {
irqnr = gic_read_iar();

@@ -525,6 +576,7 @@ static int gic_dist_supports_lpis(void)
static void gic_cpu_init(void)
{
void __iomem *rbase;
+ unsigned long __maybe_unused nmimask, hwirq;

/* Register ourselves with the rest of the world */
if (gic_populate_rdist())
@@ -545,6 +597,16 @@ static void gic_cpu_init(void)

/* initialise system registers */
gic_cpu_sys_reg_init();
+
+#ifdef CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS
+ /* Boost the priority of any IPI in the mask */
+ nmimask = SMP_IPI_NMI_MASK;
+ for_each_set_bit(hwirq, &nmimask, 16)
+ writeb_relaxed(GICD_INT_DEF_PRI ^ BIT(7),
+ rbase + GIC_DIST_PRI + hwirq);
+ gic_dist_wait_for_rwp();
+ gic_redist_wait_for_rwp();
+#endif
}

#ifdef CONFIG_SMP
diff --git a/include/linux/irqchip/arm-gic-common.h b/include/linux/irqchip/arm-gic-common.h
index c647b0547bcd..e25e1818f163 100644
--- a/include/linux/irqchip/arm-gic-common.h
+++ b/include/linux/irqchip/arm-gic-common.h
@@ -13,6 +13,14 @@
#include <linux/types.h>
#include <linux/ioport.h>

+#define GIC_DIST_PRI 0x400
+
+#define GICD_INT_DEF_PRI 0xc0
+#define GICD_INT_DEF_PRI_X4 ((GICD_INT_DEF_PRI << 24) |\
+ (GICD_INT_DEF_PRI << 16) |\
+ (GICD_INT_DEF_PRI << 8) |\
+ GICD_INT_DEF_PRI)
+
enum gic_type {
GIC_V2,
GIC_V3,
diff --git a/include/linux/irqchip/arm-gic.h b/include/linux/irqchip/arm-gic.h
index ba9ed0a2ac09..a5cc1c768e24 100644
--- a/include/linux/irqchip/arm-gic.h
+++ b/include/linux/irqchip/arm-gic.h
@@ -54,11 +54,6 @@
#define GICD_INT_EN_CLR_X32 0xffffffff
#define GICD_INT_EN_SET_SGI 0x0000ffff
#define GICD_INT_EN_CLR_PPI 0xffff0000
-#define GICD_INT_DEF_PRI 0xc0
-#define GICD_INT_DEF_PRI_X4 ((GICD_INT_DEF_PRI << 24) |\
- (GICD_INT_DEF_PRI << 16) |\
- (GICD_INT_DEF_PRI << 8) |\
- GICD_INT_DEF_PRI)

#define GICH_HCR 0x0
#define GICH_VTR 0x4
--
2.7.4

Next message: Sudeep Holla: "Re: [PATCH 03/13] scpi: Add legacy send, prepare and handle remote functions"
Previous message: Daniel Thompson: "[RFC PATCH v3 5/7] arm64: irqflags: Reorder the fiq & async macros"
In reply to: Daniel Thompson: "[RFC PATCH v3 5/7] arm64: irqflags: Reorder the fiq & async macros"
Next in thread: Daniel Thompson: "[RFC PATCH v3 4/7] arm64: alternative: Apply alternatives early in boot process"
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