[PATCH] x86, amd: Set X86_FEATURE_EXTD_APICID for future processors

[Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>]

Decision to use a 4-bit mask or 8-bit mask in default_get_apic_id()
is controlled by setting capability bit X86_FEATURE_EXTD_APICID.

Currently, we detect extended APIC ID support by accessing Link
Transaction Control register D18F0x68 in PCI config space.

But, not even that is needed as we can safely postulate that future
AMD processors will support 8-bit APIC IDs and we can simply set that
feature bit on them, without the PCI access.

Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>
---
 arch/x86/kernel/cpu/amd.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index fd470eb..20b6813 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -520,8 +520,16 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 			set_cpu_cap(c, X86_FEATURE_K6_MTRR);
 #endif
 #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_PCI)
-	/* check CPU config space for extended APIC ID */
-	if (cpu_has_apic && c->x86 >= 0xf) {
+	/*
+	 * ApicID can always be treated as a 8bit value for
+	 * AMD APIC versions >= 0x10. So, we can safely set
+	 * X86_FEATURE_EXTD_APICID unconditionally for
+	 * families after 16h.
+	 */
+	if (cpu_has_apic && c->x86 > 0x16) {
+		set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
+	} else if (cpu_has_apic && c->x86 >= 0xf) {
+		/* check CPU config space for extended APIC ID */
 		unsigned int val;
 		val = read_pci_config(0, 24, 0, 0x68);
 		if ((val & ((1 << 17) | (1 << 18))) == ((1 << 17) | (1 << 18)))
-- 
1.9.1