From 60dcaad5736faff5a6b1abba5a292499f57197fe Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Wed, 24 Jul 2019 17:25:52 +0200 Subject: x86/hotplug: Silence APIC and NMI when CPU is dead In order to support IPI/NMI broadcasting via the shorthand mechanism side effects of shorthands need to be mitigated: Shorthand IPIs and NMIs hit all CPUs including unplugged CPUs Neither of those can be handled on unplugged CPUs for obvious reasons. It would be trivial to just fully disable the APIC via the enable bit in MSR_APICBASE. But that's not possible because clearing that bit on systems based on the 3 wire APIC bus would require a hardware reset to bring it back as the APIC would lose track of bus arbitration. On systems with FSB delivery APICBASE could be disabled, but it has to be guaranteed that no interrupt is sent to the APIC while in that state and it's not clear from the SDM whether it still responds to INIT/SIPI messages. Therefore stay on the safe side and switch the APIC into soft disabled mode so it won't deliver any regular vector to the CPU. NMIs are still propagated to the 'dead' CPUs. To mitigate that add a check for the CPU being offline on early nmi entry and if so bail. Note, this cannot use the stop/restart_nmi() magic which is used in the alternatives code. A dead CPU cannot invoke nmi_enter() or anything else due to RCU and other reasons. Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1907241723290.1791@nanos.tec.linutronix.de --- arch/x86/kernel/nmi.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'arch/x86/kernel/nmi.c') diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index 4df7705022b9..e676a9916c49 100644 --- a/arch/x86/kernel/nmi.c +++ b/arch/x86/kernel/nmi.c @@ -512,6 +512,9 @@ NOKPROBE_SYMBOL(is_debug_stack); dotraplinkage notrace void do_nmi(struct pt_regs *regs, long error_code) { + if (IS_ENABLED(CONFIG_SMP) && cpu_is_offline(smp_processor_id())) + return; + if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) { this_cpu_write(nmi_state, NMI_LATCHED); return; -- cgit v1.2.3