[PATCH 4.1 77/78] x86/apic: Fix fallout from x2apic cleanup

Greg Kroah-Hartman
2015-09-11T23:00:48Z

From: Thomas Gleixner <tglx@linutronix.de>

commit a57e456a7b28431b55e407e5ab78ebd5b378d19e upstream.

In the recent x2apic cleanup I got two things really wrong:
1) The safety check in __disable_x2apic which allows the function to
be called unconditionally is backwards. The check is there to
prevent access to the apic MSR in case that the machine has no
apic. Though right now it returns if the machine has an apic and
therefor the disabling of x2apic is never invoked.

2) x2apic_disable() sets x2apic_mode to 0 after registering the local
apic. That's wrong, because register_lapic_address() checks x2apic
mode and therefor takes the wrong code path.

This results in boot failures on machines with x2apic preenabled by
BIOS and can also lead to an fatal MSR access on machines without
apic.

The solutions are simple:
1) Correct the sanity check for apic availability
2) Clear x2apic_mode _before_ calling register_lapic_address()

Fixes: 659006bf3ae3 'x86/x2apic: Split enable and setup function'
Reported-and-tested-by: Javier Monteagudo <javiermon@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1224764
Cc: Laura Abbott <labbott@redhat.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kernel/apic/apic.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1424,7 +1424,7 @@ static inline void __x2apic_disable(void
{
u64 msr;

- if (cpu_has_apic)
+ if (!cpu_has_apic)
return;

rdmsrl(MSR_IA32_APICBASE, msr);
@@ -1483,10 +1483,13 @@ void x2apic_setup(void)

static __init void x2apic_disable(void)
{
- u32 x2apic_id;
+ u32 x2apic_id, state = x2apic_state;

- if (x2apic_state != X2APIC_ON)
- goto out;
+ x2apic_mode = 0;
+ x2apic_state = X2APIC_DISABLED;
+
+ if (state != X2APIC_ON)
+ return;

x2apic_id = read_apic_id();
if (x2apic_id >= 255)
@@ -1494,9 +1497,6 @@ static __init void x2apic_disable(void)

__x2apic_disable();
register_lapic_address(mp_lapic_addr);
-out:
- x2apic_state = X2APIC_DISABLED;
- x2apic_mode = 0;
}

static __init void x2apic_enable(void)