x86/apic: Do not initialize LDR and DFR for bigsmp

Legacy apic init uses bigsmp for smp systems with 8 and more CPUs. The
bigsmp APIC implementation uses physical destination mode, but it
nevertheless initializes LDR and DFR. The LDR even ends up incorrectly with
multiple bit being set.

This does not cause a functional problem because LDR and DFR are ignored
when physical destination mode is active, but it triggered a problem on a
32-bit KVM guest which jumps into a kdump kernel.

The multiple bits set unearthed a bug in the KVM APIC implementation. The
code which creates the logical destination map for VCPUs ignores the
disabled state of the APIC and ends up overwriting an existing valid entry
and as a result, APIC calibration hangs in the guest during kdump
initialization.

Remove the bogus LDR/DFR initialization.

This is not intended to work around the KVM APIC bug. The LDR/DFR
ininitalization is wrong on its own.

The issue goes back into the pre git history. The fixes tag is the commit
in the bitkeeper import which introduced bigsmp support in 2003.

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

Fixes: db7b9e9f26b8 ("[PATCH] Clustered APIC setup for >8 CPU systems")
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190826101513.5080-2-bsd@redhat.com

diff --git a/arch/x86/kernel/apic/bigsmp_32.c b/arch/x86/kernel/apic/bigsmp_32.c
index afee386ff711e95132dd5c01e79317ac22bd9a56..caedd8d60d3610b8b9bc11f5669de16521b3ac2a 100644 (file)
--- 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)