x86/apic: Serialize LVTT and TSC_DEADLINE writes

The APIC LVTT register is MMIO mapped but the TSC_DEADLINE register is an
MSR. The write to the TSC_DEADLINE MSR is not serializing, so it's not
guaranteed that the write to LVTT has reached the APIC before the
TSC_DEADLINE MSR is written. In such a case the write to the MSR is
ignored and as a consequence the local timer interrupt never fires.

The SDM decribes this issue for xAPIC and x2APIC modes. The
serialization methods recommended by the SDM differ.

xAPIC:
 "1. Memory-mapped write to LVT Timer Register, setting bits 18:17 to 10b.
  2. WRMSR to the IA32_TSC_DEADLINE MSR a value much larger than current time-stamp counter.
  3. If RDMSR of the IA32_TSC_DEADLINE MSR returns zero, go to step 2.
  4. WRMSR to the IA32_TSC_DEADLINE MSR the desired deadline."

x2APIC:
 "To allow for efficient access to the APIC registers in x2APIC mode,
  the serializing semantics of WRMSR are relaxed when writing to the
  APIC registers. Thus, system software should not use 'WRMSR to APIC
  registers in x2APIC mode' as a serializing instruction. Read and write
  accesses to the APIC registers will occur in program order. A WRMSR to
  an APIC register may complete before all preceding stores are globally
  visible; software can prevent this by inserting a serializing
  instruction, an SFENCE, or an MFENCE before the WRMSR."

The xAPIC method is to just wait for the memory mapped write to hit
the LVTT by checking whether the MSR write has reached the hardware.
There is no reason why a proper MFENCE after the memory mapped write would
not do the same. Andi Kleen confirmed that MFENCE is sufficient for the
xAPIC case as well.

Issue MFENCE before writing to the TSC_DEADLINE MSR. This can be done
unconditionally as all CPUs which have TSC_DEADLINE also have MFENCE
support.

[ tglx: Massaged the changelog ]
Signed-off-by: Shaohua Li <shli@fb.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: <Kernel-team@fb.com>
Cc: <lenb@kernel.org>
Cc: <fenghua.yu@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: stable@vger.kernel.org #v3.7+
Link: http://lkml.kernel.org/r/20150909041352.GA2059853@devbig257.prn2.facebook.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic/apic.c

@@ -336,6 +336,13 @@ static void __setup_APIC_LVTT(unsigned int clocks, int oneshot, int irqen)
 	apic_write(APIC_LVTT, lvtt_value);
 
 	if (lvtt_value & APIC_LVT_TIMER_TSCDEADLINE) {
+		/*
+		 * See Intel SDM: TSC-Deadline Mode chapter. In xAPIC mode,
+		 * writing to the APIC LVTT and TSC_DEADLINE MSR isn't serialized.
+		 * According to Intel, MFENCE can do the serialization here.
+		 */
+		asm volatile("mfence" : : : "memory");
+
 		printk_once(KERN_DEBUG "TSC deadline timer enabled\n");
 		return;
 	}