[S] Change in coreboot[master]: cpu/x86: Add some notes about XAPIC/X2APIC

[Kyösti Mälkki (Code Review)]

Kyösti Mälkki has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/76196?usp=email )

Change subject: cpu/x86: Add some notes about XAPIC/X2APIC
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cpu/x86: Add some notes about XAPIC/X2APIC

At the time of writing SMM runtime does not make register
accesses to LAPIC registers, but such breakage has been
reported.

S3 resume failure, where OS switched back from X2APIC
to XAPIC mode, can be reproduced with a sandybridge SKU
that has VT-d disabled.

Change-Id: I300ba87c3d8fde548dbaf95703bd7e2fe54cff57
Signed-off-by: Kyösti Mälkki 
---
M src/cpu/x86/Kconfig
1 file changed, 10 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/96/76196/1

diff --git a/src/cpu/x86/Kconfig b/src/cpu/x86/Kconfig
index 1e1b2ac..4e17690 100644
--- a/src/cpu/x86/Kconfig
+++ b/src/cpu/x86/Kconfig
@@ -50,16 +50,26 @@
 config XAPIC_ONLY
    prompt "Set XAPIC mode"
    bool
+	help
+	  coreboot and SMM runtime only use XAPIC mode.
+	  FIXME: DMAR should have X2APIC optout bit set.

config X2APIC_ONLY
    prompt "Set X2APIC mode"
    bool
    depends on PARALLEL_MP
+	help
+	  coreboot and SMM runtime only use X2APIC mode.
+	  Note: OS switches back to XAPIC mode if VT-d is disabled.
+	  FIXME: S3 resume (and SMM runtime) will break if OS makes the switch.

config X2APIC_RUNTIME
    prompt "Support both XAPIC and X2APIC"
    bool
    depends on PARALLEL_MP
+	help
+	  The switch to X2APIC mode happens early in ramstage. SMM runtime can
+	  support either mode in case the OS switches back to XAPIC.

config X2APIC_LATE_WORKAROUND
    prompt "Use XAPIC for AP bringup, then change to X2APIC"