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    Subject[PATCH 4.19 49/93] x86/apic: Do not initialize LDR and DFR for bigsmp
    Date
    From: Bandan Das <bsd@redhat.com>

    commit bae3a8d3308ee69a7dbdf145911b18dfda8ade0d upstream.

    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
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/apic/bigsmp_32.c | 24 ++----------------------
    1 file changed, 2 insertions(+), 22 deletions(-)

    --- 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(i
    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)

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