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Date:   Fri, 24 May 2019 16:56:02 -0700
From:   Fenghua Yu <>
To:     "Thomas Gleixner" <>,
        "Ingo Molnar" <>, "Borislav Petkov" <>,
        "H Peter Anvin" <>,
        "Andy Lutomirski" <>,
        "Andrew Cooper" <>,
        "Ashok Raj" <>,
        "Tony Luck" <>,
        "Ravi V Shankar" <>
Cc:     "linux-kernel" <>,
        "x86" <>, Fenghua Yu <>
Subject: [PATCH v3 5/5] x86/umwait: Document umwait control sysfs interfaces

Since two new sysfs interface files are created for umwait control, add
an ABI document entry for the files:

Signed-off-by: Fenghua Yu <>
Reviewed-by: Ashok Raj <>
 .../ABI/testing/sysfs-devices-system-cpu      | 21 +++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 1528239f69b2..bbf65ae447ff 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -538,3 +538,24 @@ Description:	Intel Energy and Performance Bias Hint (EPB)
 		This attribute is present for all online CPUs supporting the
 		Intel EPB feature.
+What:		/sys/devices/system/cpu/umwait_control
+		/sys/devices/system/cpu/umwait_control/enable_c0_2
+		/sys/devices/system/cpu/umwait_control/max_time
+Date:		May 2019
+Contact:	Linux kernel mailing list <>
+Description:	Umwait control
+		enable_c0_2: Read/write interface to control umwait C0.2 state
+			Read returns C0.2 state status:
+				0: C0.2 is disabled
+				1: C0.2 is enabled
+			Write 'Yy1' or [oO][nN] for on to enable C0.2 state.
+			Write 'Nn0' or [oO][fF] for off to disable C0.2 state.
+		max_time: Read/write interface to control umwait maximum time
+			  in TSC-quanta that the CPU can reside in either C0.1
+			  or C0.2 state. The time is represented as an unsigned
+			  integer decimal value. Bits[1:0] are ignore.
+			  A zero value indicates no maximum time.

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