Revert "x86/apic: Ignore secondary threads if nosmt=force"

commit 506a66f374891ff08e064a058c446b336c5ac760 upstream

Dave Hansen reported, that it's outright dangerous to keep SMT siblings
disabled completely so they are stuck in the BIOS and wait for SIPI.

The reason is that Machine Check Exceptions are broadcasted to siblings and
the soft disabled sibling has CR4.MCE = 0. If a MCE is delivered to a
logical core with CR4.MCE = 0, it asserts IERR#, which shuts down or
reboots the machine. The MCE chapter in the SDM contains the following
blurb:

    Because the logical processors within a physical package are tightly
    coupled with respect to shared hardware resources, both logical
    processors are notified of machine check errors that occur within a
    given physical processor. If machine-check exceptions are enabled when
    a fatal error is reported, all the logical processors within a physical
    package are dispatched to the machine-check exception handler. If
    machine-check exceptions are disabled, the logical processors enter the
    shutdown state and assert the IERR# signal. When enabling machine-check
    exceptions, the MCE flag in control register CR4 should be set for each
    logical processor.

Reverting the commit which ignores siblings at enumeration time solves only
half of the problem. The core cpuhotplug logic needs to be adjusted as
well.

This thoughtful engineered mechanism also turns the boot process on all
Intel HT enabled systems into a MCE lottery. MCE is enabled on the boot CPU
before the secondary CPUs are brought up. Depending on the number of
physical cores the window in which this situation can happen is smaller or
larger. On a HSW-EX it's about 750ms:

MCE is enabled on the boot CPU:

[    0.244017] mce: CPU supports 22 MCE banks

The corresponding sibling #72 boots:

[    1.008005] .... node  #0, CPUs:    #72

That means if an MCE hits on physical core 0 (logical CPUs 0 and 72)
between these two points the machine is going to shutdown. At least it's a
known safe state.

It's obvious that the early boot can be hit by an MCE as well and then runs
into the same situation because MCEs are not yet enabled on the boot CPU.
But after enabling them on the boot CPU, it does not make any sense to
prevent the kernel from recovering.

Adjust the nosmt kernel parameter documentation as well.

Reverts: 2207def700f9 ("x86/apic: Ignore secondary threads if nosmt=force")
Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 38d75cc78a3dece4c5ed8593945ccfc538339eb2..9f3ffe23a7c9741128b0536ab5e66d6767c94239 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2596,12 +2596,8 @@
                        Equivalent to smt=1.
 
                        [KNL,x86] Disable symmetric multithreading (SMT).
-                       nosmt=force: Force disable SMT, similar to disabling
-                                    it in the BIOS except that some of the
-                                    resource partitioning effects which are
-                                    caused by having SMT enabled in the BIOS
-                                    cannot be undone. Depending on the CPU
-                                    type this might have a performance impact.
+                       nosmt=force: Force disable SMT, cannot be undone
+                                    via the sysfs control file.
 
        nospectre_v2    [X86] Disable all mitigations for the Spectre variant 2
                        (indirect branch prediction) vulnerability. System may

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 1430522b2b40ae9f682175a7e7f485807bd5ab0f..77ffd3b7894f5807f8cd5752317f0ee84a413472 100644 (file)
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -616,10 +616,8 @@ extern int default_check_phys_apicid_present(int phys_apicid);
 
 #ifdef CONFIG_SMP
 bool apic_id_is_primary_thread(unsigned int id);
-bool apic_id_disabled(unsigned int id);
 #else
 static inline bool apic_id_is_primary_thread(unsigned int id) { return false; }
-static inline bool apic_id_disabled(unsigned int id) { return false; }
 #endif
 
 extern void irq_enter(void);

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 65afb2f1cd1a96a67be9a2eb94156e506877a8db..6dda3595acf8571b750fb58236e889a5e991396f 100644 (file)
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -181,8 +181,7 @@ static int acpi_register_lapic(int id, u32 acpiid, u8 enabled)
        }
 
        if (!enabled) {
-               if (!apic_id_disabled(id))
-                       ++disabled_cpus;
+               ++disabled_cpus;
                return -EINVAL;
        }
 

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 828ee43abd9116093bcdd2dfa458b9caabcd378a..0a10d3ed77cebc7b911ec7ddc7b632d4ebe43b3f 100644 (file)
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2107,16 +2107,6 @@ bool apic_id_is_primary_thread(unsigned int apicid)
        return !(apicid & mask);
 }
 
-/**
- * apic_id_disabled - Check whether APIC ID is disabled via SMT control
- * @id:        APIC ID to check
- */
-bool apic_id_disabled(unsigned int id)
-{
-       return (cpu_smt_control == CPU_SMT_FORCE_DISABLED &&
-               !apic_id_is_primary_thread(id));
-}
-
 /*
  * Should use this API to allocate logical CPU IDs to keep nr_logical_cpuids
  * and cpuid_to_apicid[] synchronized.
@@ -2212,15 +2202,6 @@ int generic_processor_info(int apicid, int version)
                return -EINVAL;
        }
 
-       /*
-        * If SMT is force disabled and the APIC ID belongs to
-        * a secondary thread, ignore it.
-        */
-       if (apic_id_disabled(apicid)) {
-               pr_info_once("Ignoring secondary SMT threads\n");
-               return -EINVAL;
-       }
-
        if (apicid == boot_cpu_physical_apicid) {
                /*
                 * x86_bios_cpu_apicid is required to have processors listed