From: Russell King rmk+kernel@armlinux.org.uk
[ Upstream commit 5388a5b82199facacd3d7ac0d05aca6e8f902fed ]
machine_crash_nonpanic_core() does this:
while (1) cpu_relax();
because the kernel has crashed, and we have no known safe way to deal with the CPU. So, we place the CPU into an infinite loop which we expect it to never exit - at least not until the system as a whole is reset by some method.
In the absence of erratum 754327, this code assembles to:
b .
In other words, an infinite loop. When erratum 754327 is enabled, this becomes:
1: dmb b 1b
It has been observed that on some systems (eg, OMAP4) where, if a crash is triggered, the system tries to kexec into the panic kernel, but fails after taking the secondary CPU down - placing it into one of these loops. This causes the system to livelock, and the most noticable effect is the system stops after issuing:
Loading crashdump kernel...
to the system console.
The tested as working solution I came up with was to add wfe() to these infinite loops thusly:
while (1) { cpu_relax(); wfe(); }
which, without 754327 builds to:
1: wfe b 1b
or with 754327 is enabled:
1: dmb wfe b 1b
Adding "wfe" does two things depending on the environment we're running under: - where we're running on bare metal, and the processor implements "wfe", it stops us spinning endlessly in a loop where we're never going to do any useful work. - if we're running in a VM, it allows the CPU to be given back to the hypervisor and rescheduled for other purposes (maybe a different VM) rather than wasting CPU cycles inside a crashed VM.
However, in light of erratum 794072, Will Deacon wanted to see 10 nops as well - which is reasonable to cover the case where we have erratum 754327 enabled _and_ we have a processor that doesn't implement the wfe hint.
So, we now end up with:
1: wfe b 1b
when erratum 754327 is disabled, or:
1: dmb nop nop nop nop nop nop nop nop nop nop wfe b 1b
when erratum 754327 is enabled. We also get the dmb + 10 nop sequence elsewhere in the kernel, in terminating loops.
This is reasonable - it means we get the workaround for erratum 794072 when erratum 754327 is enabled, but still relinquish the dead processor - either by placing it in a lower power mode when wfe is implemented as such or by returning it to the hypervisior, or in the case where wfe is a no-op, we use the workaround specified in erratum 794072 to avoid the problem.
These as two entirely orthogonal problems - the 10 nops addresses erratum 794072, and the wfe is an optimisation that makes the system more efficient when crashed either in terms of power consumption or by allowing the host/other VMs to make use of the CPU.
I don't see any reason not to use kexec() inside a VM - it has the potential to provide automated recovery from a failure of the VMs kernel with the opportunity for saving a crashdump of the failure. A panic() with a reboot timeout won't do that, and reading the libvirt documentation, setting on_reboot to "preserve" won't either (the documentation states "The preserve action for an on_reboot event is treated as a destroy".) Surely it has to be a good thing to avoiding having CPUs spinning inside a VM that is doing no useful work.
Acked-by: Will Deacon will.deacon@arm.com Signed-off-by: Russell King rmk+kernel@armlinux.org.uk Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm/include/asm/barrier.h | 2 ++ arch/arm/include/asm/processor.h | 6 +++++- arch/arm/kernel/machine_kexec.c | 5 ++++- arch/arm/kernel/smp.c | 4 +++- arch/arm/mach-omap2/prm_common.c | 4 +++- 5 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h index 3ff5642d9788..27c1d26b05b5 100644 --- a/arch/arm/include/asm/barrier.h +++ b/arch/arm/include/asm/barrier.h @@ -10,6 +10,8 @@ #define sev() __asm__ __volatile__ ("sev" : : : "memory") #define wfe() __asm__ __volatile__ ("wfe" : : : "memory") #define wfi() __asm__ __volatile__ ("wfi" : : : "memory") +#else +#define wfe() do { } while (0) #endif
#if __LINUX_ARM_ARCH__ >= 7 diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h index 8a1e8e995dae..08509183c7df 100644 --- a/arch/arm/include/asm/processor.h +++ b/arch/arm/include/asm/processor.h @@ -77,7 +77,11 @@ extern void release_thread(struct task_struct *); unsigned long get_wchan(struct task_struct *p);
#if __LINUX_ARM_ARCH__ == 6 || defined(CONFIG_ARM_ERRATA_754327) -#define cpu_relax() smp_mb() +#define cpu_relax() \ + do { \ + smp_mb(); \ + __asm__ __volatile__("nop; nop; nop; nop; nop; nop; nop; nop; nop; nop;"); \ + } while (0) #else #define cpu_relax() barrier() #endif diff --git a/arch/arm/kernel/machine_kexec.c b/arch/arm/kernel/machine_kexec.c index 8bf3b7c09888..46519916a465 100644 --- a/arch/arm/kernel/machine_kexec.c +++ b/arch/arm/kernel/machine_kexec.c @@ -87,8 +87,11 @@ void machine_crash_nonpanic_core(void *unused)
set_cpu_online(smp_processor_id(), false); atomic_dec(&waiting_for_crash_ipi); - while (1) + + while (1) { cpu_relax(); + wfe(); + } }
static void machine_kexec_mask_interrupts(void) diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c index 08ce9e36dc5a..0f1c11861147 100644 --- a/arch/arm/kernel/smp.c +++ b/arch/arm/kernel/smp.c @@ -563,8 +563,10 @@ static void ipi_cpu_stop(unsigned int cpu) local_fiq_disable(); local_irq_disable();
- while (1) + while (1) { cpu_relax(); + wfe(); + } }
static DEFINE_PER_CPU(struct completion *, cpu_completion); diff --git a/arch/arm/mach-omap2/prm_common.c b/arch/arm/mach-omap2/prm_common.c index 0ce4548ef7f0..4b9e9d1d8229 100644 --- a/arch/arm/mach-omap2/prm_common.c +++ b/arch/arm/mach-omap2/prm_common.c @@ -533,8 +533,10 @@ void omap_prm_reset_system(void)
prm_ll_data->reset_system();
- while (1) + while (1) { cpu_relax(); + wfe(); + } }
/**