linux-kernel - [PATCH v2 1/2] x86/apic: Do not initialize LDR and DFR for bigsmp

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Date:   Mon, 26 Aug 2019 06:15:12 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] x86/apic: Do not initialize LDR and DFR for bigsmp

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@...utronix.de>
Signed-off-by: Bandan Das <bsd@...hat.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