• Bandan Das's avatar
    x86/apic: Do not initialize LDR and DFR for bigsmp · bae3a8d3
    Bandan Das authored
    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: db7b9e9f ("[PATCH] Clustered APIC setup for >8 CPU systems")
    Suggested-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarBandan Das <bsd@redhat.com>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/20190826101513.5080-2-bsd@redhat.com
    
    bae3a8d3
bigsmp_32.c 4.44 KB