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Subject[PATCH 3/4] x86/smp: send capped cpus to a stable state when smp_init()
Date
*** Back ground ***

On x86 it's required to have all logical CPUs set CR4.MCE=1. Otherwise, a
broadcast MCE observing CR4.MCE=0b on any core will shutdown the machine.

The option 'nosmt' has already complied with the above rule by Thomas's
patch. For detail, refer to 506a66f3748 (Revert "x86/apic: Ignore secondary
threads if nosmt=force")

But for nr_cpus option, the exposure to broadcast MCE is a little
complicated, and can be categorized into three cases.

-1. boot up by BIOS. Since no one set CR4.MCE=1, nr_cpus risks rebooting by
broadcast MCE.

-2. boot up by "kexec -p nr_cpus=". Since the 1st kernel has all cpus'
CR4.MCE=1 set before kexec -p, nr_cpus is free of rebooting by broadcast
MCE. Furthermore, the crashed kernel's wreckage, including page table and
text, is not touched by capture kernel. Hence if MCE event happens on
capped cpu, do_machine_check->__mc_check_crashing_cpu() runs smoothly and
returns immediately, the capped cpu is still pinned on "halt".

-3. boot up by "kexec -l nr_cpus=". As "kexec -p", it is free of rebooting
by broadcast MCE. But the 1st kernel's wreckage is discarded and changed.
when capped cpus execute do_machine_check(), they may crack the new kernel.
But this is not related with broadcast MCE, and need an extra fix.

In a word, only case 1 suffers from unexpected rebooting due to broadcast
MCE.

*** Solution ***

When fixing case 1, "nr_cpus" can not follow the same way as "nosmt".
Because nr_cpus limits the allocation of percpu area and some other kthread
memory, which is critical to cpu hotplug framework.

Instead, developing a dedicated SIPI callback make_capped_cpu_stable() for
capped cpu, which does not lean on percpu area to work.

BTW this patch has side effect to suppress case 3. It brings capped cpu to
make_capped_cpu_stable() in the 2nd kernel's context, instead of leaving
them with the 1st kernel's context.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
To: x86@kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Daniel Drake <drake@endlessm.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: linux-kernel@vger.kernel.org
Cc: Dave Young <dyoung@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: kexec@lists.infradead.org
---
arch/x86/include/asm/apic.h | 1 +
arch/x86/include/asm/smp.h | 1 +
arch/x86/kernel/apic/apic.c | 2 +-
arch/x86/kernel/cpu/common.c | 3 ++
arch/x86/kernel/smpboot.c | 83 ++++++++++++++++++++++++++++++++++++++++++++
kernel/smp.c | 6 ++++
6 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index e647aa0..02232e3 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -49,6 +49,7 @@ static inline void generic_apic_probe(void)

#ifdef CONFIG_X86_LOCAL_APIC

+extern int cpuid_to_apicid[];
extern int apic_verbosity;
extern int local_apic_timer_c2_ok;

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 5f63399..f67ce77 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -197,6 +197,7 @@ extern void nmi_selftest(void);
#endif

extern struct cpumask *cpu_capped_mask;
+extern struct cpumask *cpu_capped_done_mask;

#endif /* __ASSEMBLY__ */
#endif /* _ASM_X86_SMP_H */
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 6a57bad3..b86c9e6 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2244,7 +2244,7 @@ static int nr_logical_cpuids = 1;
/*
* Used to store mapping between logical CPU IDs and APIC IDs.
*/
-static int cpuid_to_apicid[] = {
+int cpuid_to_apicid[] = {
[0 ... NR_CPUS - 1] = -1,
};

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4d87df5..1a62b37 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -69,6 +69,8 @@ cpumask_var_t cpu_callin_mask;
/* size of NR_CPUS is required. */
struct cpumask __cpu_capped_mask __initdata;
struct cpumask *cpu_capped_mask;
+struct cpumask __cpu_capped_done_mask __initdata;
+struct cpumask *cpu_capped_done_mask;

/* representing cpus for which sibling maps can be computed */
cpumask_var_t cpu_sibling_setup_mask;
@@ -88,6 +90,7 @@ void __init setup_cpu_local_masks(void)
alloc_bootmem_cpumask_var(&cpu_callout_mask);
alloc_bootmem_cpumask_var(&cpu_sibling_setup_mask);
cpu_capped_mask = &__cpu_capped_mask;
+ cpu_capped_done_mask = &__cpu_capped_done_mask;
}

static void default_init(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index fdbd47c..d247fbd 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1014,6 +1014,89 @@ int common_cpu_up(unsigned int cpu, struct task_struct *idle)
return 0;
}

+/* SIPI callback for the capped cpu */
+static void notrace make_capped_cpu_stable(void *unused)
+{
+ unsigned int apic_id;
+ int i, cpu = -1;
+
+ /* protect against MCE broadcast */
+ native_write_cr4(X86_CR4_MCE);
+ x2apic_setup();
+ apic_id = read_apic_id();
+ for (i = 0; i < NR_CPUS; i++)
+ if (cpuid_to_apicid[i] == apic_id) {
+ cpu = i;
+ break;
+ }
+
+ /* trampoline_*.S has loaded idt with 0, 0 */
+
+ /* No response to any interrupt */
+ disable_local_APIC();
+ /* Done with the temporary stack, signal the master to go ahead */
+ if (cpu != -1) {
+ cpumask_clear_cpu(cpu, cpu_capped_mask);
+ cpumask_set_cpu(cpu, cpu_capped_done_mask);
+ }
+
+ /* run without stack and can not have the __init attribute */
+ do {
+ asm volatile("hlt" : : : "memory");
+ } while (true);
+}
+
+static void __init do_stable_cpu(int cpu)
+{
+ static char capped_tmp_stack[512];
+ int cpu0_nmi_registered = 0, apicid = cpuid_to_apicid[cpu];
+ unsigned long start_ip = real_mode_header->trampoline_start;
+ unsigned long timeout, boot_error = 0;
+
+ /* invalid percpu area */
+ initial_gs = 0;
+ /*
+ * Borrow the value of cpu 0. Since capped cpu segment shadow register
+ * can cache the content, and keep it unchanged.
+ */
+ early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(0);
+ initial_code = (unsigned long)make_capped_cpu_stable;
+ initial_stack = (unsigned long)&capped_tmp_stack;
+
+ if (apic->wakeup_secondary_cpu)
+ boot_error = apic->wakeup_secondary_cpu(apicid, start_ip);
+ else
+ boot_error = wakeup_cpu_via_init_nmi(cpu, start_ip, apicid,
+ &cpu0_nmi_registered);
+ if (cpu0_nmi_registered)
+ unregister_nmi_handler(NMI_LOCAL, "wake_cpu0");
+
+ if (!boot_error) {
+ /* Wait 10s total for first sign of life from capped cpu */
+ boot_error = -1;
+ timeout = jiffies + 10*HZ;
+ while (time_before(jiffies, timeout)) {
+ if (cpumask_test_cpu(cpu, cpu_capped_done_mask))
+ break;
+ schedule();
+ }
+ }
+}
+
+void __init bring_capped_cpu_stable(void)
+{
+ int cpu;
+
+ /* Guest does not suffer from MCE broadcast */
+ if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
+ return;
+
+ for_each_cpu(cpu, cpu_capped_mask)
+ do_stable_cpu(cpu);
+ cpu_capped_mask = NULL;
+ cpu_capped_done_mask = NULL;
+}
+
/*
* NOTE - on most systems this is a PHYSICAL apic ID, but on multiquad
* (ie clustered apic addressing mode), this is a LOGICAL apic ID.
diff --git a/kernel/smp.c b/kernel/smp.c
index 7dbcb40..de528b7 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -574,6 +574,10 @@ void __init setup_nr_cpu_ids(void)
nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
}

+void __weak __init bring_capped_cpu_stable(void)
+{
+}
+
/* Called by boot processor to activate the rest. */
void __init smp_init(void)
{
@@ -593,6 +597,8 @@ void __init smp_init(void)
cpu_up(cpu);
}

+ /* force cpus capped by nr_cpus option into stable state */
+ bring_capped_cpu_stable();
num_nodes = num_online_nodes();
num_cpus = num_online_cpus();
pr_info("Brought up %d node%s, %d CPU%s\n",
--
2.7.5
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