x86/apic: Ignore secondary threads if nosmt=force

nosmt on the kernel command line merely prevents the onlining of the
secondary SMT siblings.

nosmt=force makes the APIC detection code ignore the secondary SMT siblings
completely, so they even do not show up as possible CPUs. That reduces the
amount of memory allocations for per cpu variables and saves other
resources from being allocated too large.

This is not fully equivalent to disabling SMT in the BIOS because the low
level SMT enabling in the BIOS can result in partitioning of resources
between the siblings, which is not undone by just ignoring them. Some CPUs
can use the full resources when their sibling is not onlined, but this is
depending on the CPU family and model and it's not well documented whether
this applies to all partitioned resources. That means depending on the
workload disabling SMT in the BIOS might result in better performance.

Linus analysis of the Intel manual:

  The intel optimization manual is not very clear on what the partitioning
  rules are.

  I find:

    "In general, the buffers for staging instructions between major pipe
     stages  are partitioned. These buffers include µop queues after the
     execution trace cache, the queues after the register rename stage, the
     reorder buffer which stages instructions for retirement, and the load
     and store buffers.

     In the case of load and store buffers, partitioning also provided an
     easier implementation to maintain memory ordering for each logical
     processor and detect memory ordering violations"

  but some of that partitioning may be relaxed if the HT thread is "not
  active":

    "In Intel microarchitecture code name Sandy Bridge, the micro-op queue
     is statically partitioned to provide 28 entries for each logical
     processor,  irrespective of software executing in single thread or
     multiple threads. If one logical processor is not active in Intel
     microarchitecture code name Ivy Bridge, then a single thread executing
     on that processor  core can use the 56 entries in the micro-op queue"

  but I do not know what "not active" means, and how dynamic it is. Some of
  that partitioning may be entirely static and depend on the early BIOS
  disabling of HT, and even if we park the cores, the resources will just be
  wasted.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Orabug: 28220674
CVE: CVE-2018-3620

(cherry picked from commit 2207def700f902f169fc237b717252c326f9e464)

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Conflicts:
	arch/x86/include/asm/apic.h
	arch/x86/kernel/apic/apic.c
Contextual

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 1349b2842a55ca52329fe9a637e941f0e15ceda3..d18259c1b966861a364ae0eaae7cecff6ae398df 100644 (file)
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -117,8 +117,10 @@ static inline bool apic_is_x2apic_enabled(void)
 
 #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
 
 #ifdef CONFIG_X86_X2APIC

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 1e32f9133a49af0e8f51fb474de5bd61d1e0773e..ad2e4ffed512125b93daf806439ca900c7d0da45 100644 (file)
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -177,7 +177,8 @@ static int acpi_register_lapic(int id, u32 acpiid, u8 enabled)
        }
 
        if (!enabled) {
-               ++disabled_cpus;
+               if (!apic_id_disabled(id))
+                       ++disabled_cpus;
                return -EINVAL;
        }
 

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index e900f44a36f79a0d3fae76876bb1deb36bc672a9..bd518d38764c70994d6f66291ebf91bd5c794ed0 100644 (file)
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2001,6 +2001,16 @@ 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));
+}
+
 int generic_processor_info(int apicid, int version)
 {
        int cpu, max = nr_cpu_ids;
@@ -2067,6 +2077,15 @@ 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;
+       }
+
        num_processors++;
        if (apicid == boot_cpu_physical_apicid) {
                /*