x86/timer: Force PIT initialization when !X86_FEATURE_ARAT

KVM guests with commit c8c4076 ("x86/timer: Skip PIT initialization on
modern chipsets") applied to guest kernel have been observed to have
unusually higher CPU usage with symptoms of increase in vm exits for HLT
and MSW_WRITE (MSR_IA32_TSCDEADLINE).

This is caused by older QEMUs lacking support for X86_FEATURE_ARAT.  lapic
clock retains CLOCK_EVT_FEAT_C3STOP and nohz stays inactive.  There's no
usable broadcast device either.

Do the PIT initialization if guest CPU lacks X86_FEATURE_ARAT.  On real
hardware it shouldn't matter as ARAT and DEADLINE come together.

Fixes: c8c4076 ("x86/timer: Skip PIT initialization on modern chipsets")
Signed-off-by: Jan Stancek <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

Author: jstancek

arch/x86/kernel/apic/apic.c

@@ -834,6 +834,10 @@ bool __init apic_needs_pit(void)
 	if (!boot_cpu_has(X86_FEATURE_APIC))
 		return true;
 
+	/* Virt guests may lack ARAT, but still have DEADLINE */
+	if (!boot_cpu_has(X86_FEATURE_ARAT))
+		return true;
+
 	/* Deadline timer is based on TSC so no further PIT action required */
 	if (boot_cpu_has(X86_FEATURE_TSC_DEADLINE_TIMER))
 		return false;