x86/apic: Include the LDR when clearing out APIC registers

Although APIC initialization will typically clear out the LDR before
setting it, the APIC cleanup code should reset the LDR.

This was discovered with a 32-bit KVM guest jumping into a kdump
kernel. The stale bits in the LDR triggered a bug in the KVM APIC
implementation which caused the destination mapping for VCPUs to be
corrupted.

Note that this isn't intended to paper over the KVM APIC bug. The kernel
has to clear the LDR when resetting the APIC registers except when X2APIC
is enabled.

This lacks a Fixes tag because missing to clear LDR goes way back into pre
git history.

[ tglx: Made x2apic_enabled a function call as required ]

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/20190826101513.5080-3-bsd@redhat.com

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index aa5495d0f47832fbc02d102a8fb66180a550b6e9..dba2828b779a57abe32892600be3fdc021a5254a 100644 (file)
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1179,6 +1179,10 @@ void clear_local_APIC(void)
        apic_write(APIC_LVT0, v | APIC_LVT_MASKED);
        v = apic_read(APIC_LVT1);
        apic_write(APIC_LVT1, v | APIC_LVT_MASKED);
+       if (!x2apic_enabled()) {
+               v = apic_read(APIC_LDR) & ~APIC_LDR_MASK;
+               apic_write(APIC_LDR, v);
+       }
        if (maxlvt >= 4) {
                v = apic_read(APIC_LVTPC);
                apic_write(APIC_LVTPC, v | APIC_LVT_MASKED);