path: root/Documentation
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authorSuresh Siddha <>2011-08-23 17:05:18 -0700
committerHiroshi DOYU <>2012-01-23 11:11:33 +0200
commitfb599376b7dbc5c3ffa1e4d4a2497bb352fa573e (patch)
tree7bd6c915d0746e7f41a96a4eccd2d2babc05406c /Documentation
parent7f42887a91d20ed6ebe793fb02f5b56b7aee8120 (diff)
x86, x2apic: Enable the bios request for x2apic optout
On the platforms which are x2apic and interrupt-remapping capable, Linux kernel is enabling x2apic even if the BIOS doesn't. This is to take advantage of the features that x2apic brings in. Some of the OEM platforms are running into issues because of this, as their bios is not x2apic aware. For example, this was resulting in interrupt migration issues on one of the platforms. Also if the BIOS SMI handling uses APIC interface to send SMI's, then the BIOS need to be aware of x2apic mode that OS has enabled. On some of these platforms, BIOS doesn't have a HW mechanism to turnoff the x2apic feature to prevent OS from enabling it. To resolve this mess, recent changes to the VT-d2 specification: includes a mechanism that provides BIOS a way to request system software to opt out of enabling x2apic mode. Look at the x2apic optout flag in the DMAR tables before enabling the x2apic mode in the platform. Also print a warning that we have disabled x2apic based on the BIOS request. Kernel boot parameter "intremap=no_x2apic_optout" can be used to override the BIOS x2apic optout request. Signed-off-by: Youquan Song <> Signed-off-by: Suresh Siddha <> Cc: Cc: Cc: Cc: Link: Signed-off-by: Ingo Molnar <>
Diffstat (limited to 'Documentation')
1 files changed, 2 insertions, 1 deletions
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index d6e6724446c8..73e0d3525624 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1014,10 +1014,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
has the capability. With this option, super page will
not be supported.
intremap= [X86-64, Intel-IOMMU]
- Format: { on (default) | off | nosid }
on enable Interrupt Remapping (default)
off disable Interrupt Remapping
nosid disable Source ID checking
+ no_x2apic_optout
+ BIOS x2APIC opt-out request will be ignored
inttest= [IA-64]