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Thu, 29 Jan 2026 15:48:00 +0000 Message-ID: Date: Thu, 29 Jan 2026 07:47:58 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/7] stop_machine: Introduce stop_machine_nmi() To: Borislav Petkov , "Kaplan, David" CC: "linux-kernel@vger.kernel.org" , "x86@kernel.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "dave.hansen@linux.intel.com" , "peterz@infradead.org" References: <20260125014224.249901-1-chang.seok.bae@intel.com> <20260125014224.249901-2-chang.seok.bae@intel.com> <20260127154948.GLaXjenHeJ7Ay6OFJ5@fat_crate.local> <20260127204957.GMaXkk9WRLzn89WP-n@fat_crate.local> <20260128163556.GOaXo67MJ43kCRs68m@fat_crate.local> <20260129121729.GRaXtP2aeWkQKegxC2@fat_crate.local> Content-Language: en-US From: "Chang S. 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> - > -bool stop_machine_nmi_handler_enabled(void) > -{ > - return static_branch_unlikely(&stop_machine_nmi_handler_enable); > -} Thanks for the detailed write-up! I just wanted to clarify a few points, if you don't mind: First, on patch6, it is going to be used by microcode_offline_nmi_handler() { if (!raw_cpu_read(ucode_ctrl.nmi_enabled)) return; raw_cpu_write(ucode_ctrl.nmi_enabled, false); raw_cpu_write(ucode_ctrl.result, UCODE_OFFLINE); raw_atomic_inc(&offline_in_nmi); wait_for_ctrl(); } I assume you consider the per-CPU nmi_enabled bit is enough going there. Then, DEFINE_IDTENTRY_RAW(exc_nmi) { ... if (arch_cpu_is_offline(smp_processor_id())) { microcode_offline_nmi_handler(); return; } ... } Correct? The other bit is possible bug after the removal. The NMI path effectively becomes: static noinstr void default_do_nmi(struct pt_regs *regs) ... + if (stop_machine_nmi_handler()) + goto out; which means it could be invoked from an NMI unrelated to stop-machine. > bool noinstr stop_machine_nmi_handler(void) > { > - struct multi_stop_data *msdata; > + struct multi_stop_data *msdata = raw_cpu_read(stop_machine_nmi_ctrl.msdata); In that case, msdata could still be NULL here. > int err; > > - if (!raw_cpu_read(stop_machine_nmi_ctrl.nmi_enabled)) > + if (!cpumask_test_and_clear_cpu(smp_processor_id(), &msdata->nmi_cpus)) > return false; > > - raw_cpu_write(stop_machine_nmi_ctrl.nmi_enabled, false); > - > - msdata = raw_cpu_read(stop_machine_nmi_ctrl.msdata); > instrumentation_begin(); > err = msdata->fn(msdata->data); > instrumentation_end(); Also, I suppose this_cpu_write(stop_machine_nmi_ctrl.msdata, NULL); > @@ -243,6 +232,17 @@ bool noinstr stop_machine_nmi_handler(void) > return true; > } > Finally, while I do appreciate the nmi_cpus approach, I could also think another simple msdata == NULL check could be a guard here too. Thanks, Chang