x86/apic: Enable x2APIC without interrupt remapping under KVM

KVM would like to provide x2APIC interface to a guest without emulating
interrupt remapping device. The reason KVM prefers guest to use x2APIC
is that x2APIC interface is better virtualizable and provides better
performance than mmio xAPIC interface:

 - msr exits are faster than mmio (no page table walk, emulation)
 - no need to read back ICR to look at the busy bit
 - one 64 bit ICR write instead of two 32 bit writes
 - shared code with the Hyper-V paravirt interface

Included patch changes x2APIC enabling logic to enable it even if IR
initialization failed, but kernel runs under KVM and no apic id is
greater than 255 (if there is one spec requires BIOS to move to x2apic
mode before starting an OS).

-v2: fix build
-v3: fix bug causing compiler warning

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Sheng Yang <sheng@linux.intel.com>
Cc: "avi@redhat.com" <avi@redhat.com>
LKML-Reference: <20090720122417.GR5638@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/x86/kernel/apic/probe_64.c b/arch/x86/kernel/apic/probe_64.c
index bc3e880..f3b1037 100644
--- a/arch/x86/kernel/apic/probe_64.c
+++ b/arch/x86/kernel/apic/probe_64.c
@@ -50,11 +50,11 @@ static struct apic *apic_probe[] __initdata = {
 void __init default_setup_apic_routing(void)
 {
 #ifdef CONFIG_X86_X2APIC
-	if (x2apic_mode && (apic != &apic_x2apic_phys &&
+	if (x2apic_mode
 #ifdef CONFIG_X86_UV
-		       apic != &apic_x2apic_uv_x &&
+		       && apic != &apic_x2apic_uv_x
 #endif
-		       apic != &apic_x2apic_cluster)) {
+		       ) {
 		if (x2apic_phys)
 			apic = &apic_x2apic_phys;
 		else