Re: [PATCH] x86, apic: Enable x2APIC physical when cpu < 256 native

[Youquan Song <youquan.song@linux.intel.com>]

> No problem - you might want to send another patch adding some comments to 
> the code, explaining why we don't switch to physical mode, quoting from 
> the SDM and so.

Here is the revert patch.

Subject: [PATCH] Revert "x86/apic: Enable x2APIC physical mode on native hardware too, when there are fewer than 256 CPUs"

x2APIC without interrupt remapping is not architecture and no guarantee it 
will work in future.
There are some words in SDM3, 10.12.7 Initialization by System
Software Routing of device interrupts to local APIC units operating in 
x2APIC mode requires use of the interrupt-remapping architecture 
specified in the Intel Virtualization Technology for Directed I/O, 
Revision 1.3. Because of this, BIOS must enumerate support for and 
software must enable this interrupt remapping with Extended Interrupt 
Mode Enabled before it enabling x2APIC mode in the local APIC units.

This reverts commit 3d1acb49d22fbbae96524040e9e2d4cbbb3adbef, do not use
x2apic_pysical mode if interrupt remapping is not enabled even at CPU
number fewer than 256.

Signed-off-by: Youquan Song <youquan.song@intel.com>
---
 arch/x86/kernel/apic/apic.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index d9dd5a6..eca89c5 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1622,8 +1622,11 @@ void __init enable_IR_x2apic(void)
 		goto skip_x2apic;
 
 	if (ret < 0) {
-		/* IR is required if there is APIC ID > 255 */
-		if (max_physical_apicid > 255) {
+		/* IR is required if there is APIC ID > 255 even when running
+		 * under KVM
+		 */
+		if (max_physical_apicid > 255 ||
+		    !hypervisor_x2apic_available()) {
 			if (x2apic_preenabled)
 				disable_x2apic();
 			goto skip_x2apic;
-- 
1.6.4.2