Messages in this thread Patch in this message | | | From | Bandan Das <> | Subject | [PATCH v2 1/2] x86/apic: Do not initialize LDR and DFR for bigsmp | Date | Mon, 26 Aug 2019 06:15:12 -0400 |
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Legacy apic init uses bigsmp for > 8 smp systems. In these cases, PhysFlat will invariably be used and there is no point in initializing apic LDR and DFR. Furthermore, calculate_ldr() helper function was incorrectly setting multiple bits in the LDR.
This was discovered with a 32 bit KVM guest loading the kdump kernel. Owing to the multiple bits being incorrectly set in the LDR, KVM hit a buggy "if" condition in the kvm lapic implementation that creates the logical destination map for vcpus - it ends up overwriting and existing valid entry and as a result, apic calibration hangs in the guest during kdump initialization.
Note that this change isn't intended to workaround the kvm lapic bug; bigsmp should correctly stay away from initializing LDR.
Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Bandan Das <bsd@redhat.com> --- arch/x86/kernel/apic/bigsmp_32.c | 24 ++---------------------- 1 file changed, 2 insertions(+), 22 deletions(-)
diff --git a/arch/x86/kernel/apic/bigsmp_32.c b/arch/x86/kernel/apic/bigsmp_32.c index afee386ff711..caedd8d60d36 100644 --- a/arch/x86/kernel/apic/bigsmp_32.c +++ b/arch/x86/kernel/apic/bigsmp_32.c @@ -38,32 +38,12 @@ static int bigsmp_early_logical_apicid(int cpu) return early_per_cpu(x86_cpu_to_apicid, cpu); } -static inline unsigned long calculate_ldr(int cpu) -{ - unsigned long val, id; - - val = apic_read(APIC_LDR) & ~APIC_LDR_MASK; - id = per_cpu(x86_bios_cpu_apicid, cpu); - val |= SET_APIC_LOGICAL_ID(id); - - return val; -} - /* - * Set up the logical destination ID. - * - * Intel recommends to set DFR, LDR and TPR before enabling - * an APIC. See e.g. "AP-388 82489DX User's Manual" (Intel - * document number 292116). So here it goes... + * bigsmp enables physical destination mode + * and doesn't use LDR and DFR */ static void bigsmp_init_apic_ldr(void) { - unsigned long val; - int cpu = smp_processor_id(); - - apic_write(APIC_DFR, APIC_DFR_FLAT); - val = calculate_ldr(cpu); - apic_write(APIC_LDR, val); } static void bigsmp_setup_apic_routing(void) -- 2.20.1
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