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Subject[PATCH 4.9 06/31] x86/msr: Add the IA32_TSX_CTRL MSR
From: Pawan Gupta <>

commit c2955f270a84762343000f103e0640d29c7a96f3 upstream.

Transactional Synchronization Extensions (TSX) may be used on certain
processors as part of a speculative side channel attack. A microcode
update for existing processors that are vulnerable to this attack will
add a new MSR - IA32_TSX_CTRL to allow the system administrator the
option to disable TSX as one of the possible mitigations.

The CPUs which get this new MSR after a microcode upgrade are the ones
which do not set MSR_IA32_ARCH_CAPABILITIES.MDS_NO (bit 5) because those
CPUs have CPUID.MD_CLEAR, i.e., the VERW implementation which clears all
CPU buffers takes care of the TAA case as well.

[ Note that future processors that are not vulnerable will also
support the IA32_TSX_CTRL MSR. ]

Add defines for the new IA32_TSX_CTRL MSR and its bits.

TSX has two sub-features:

1. Restricted Transactional Memory (RTM) is an explicitly-used feature
where new instructions begin and end TSX transactions.
2. Hardware Lock Elision (HLE) is implicitly used when certain kinds of
"old" style locks are used by software.

Bit 7 of the IA32_ARCH_CAPABILITIES indicates the presence of the

There are two control bits in IA32_TSX_CTRL MSR:

Bit 0: When set, it disables the Restricted Transactional Memory (RTM)
sub-feature of TSX (will force all transactions to abort on the
XBEGIN instruction).

Bit 1: When set, it disables the enumeration of the RTM and HLE feature
(i.e. it will make CPUID(EAX=7).EBX{bit4} and
CPUID(EAX=7).EBX{bit11} read as 0).

The other TSX sub-feature, Hardware Lock Elision (HLE), is
unconditionally disabled by the new microcode but still enumerated
as present by CPUID(EAX=7).EBX{bit4}, unless disabled by

Signed-off-by: Pawan Gupta <>
Signed-off-by: Borislav Petkov <>
Signed-off-by: Thomas Gleixner <>
Tested-by: Neelima Krishnan <>
Reviewed-by: Mark Gross <>
Reviewed-by: Tony Luck <>
Reviewed-by: Josh Poimboeuf <>
Signed-off-by: Ben Hutchings <>
Signed-off-by: Greg Kroah-Hartman <>
arch/x86/include/asm/msr-index.h | 5 +++++
1 file changed, 5 insertions(+)

--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -77,6 +77,7 @@
* Microarchitectural Data
* Sampling (MDS) vulnerabilities.
+#define ARCH_CAP_TSX_CTRL_MSR BIT(7) /* MSR for TSX control is available. */

#define MSR_IA32_FLUSH_CMD 0x0000010b
#define L1D_FLUSH BIT(0) /*
@@ -87,6 +88,10 @@
#define MSR_IA32_BBL_CR_CTL 0x00000119
#define MSR_IA32_BBL_CR_CTL3 0x0000011e

+#define MSR_IA32_TSX_CTRL 0x00000122
+#define TSX_CTRL_RTM_DISABLE BIT(0) /* Disable RTM feature */
+#define TSX_CTRL_CPUID_CLEAR BIT(1) /* Disable TSX enumeration */
#define MSR_IA32_SYSENTER_CS 0x00000174
#define MSR_IA32_SYSENTER_ESP 0x00000175
#define MSR_IA32_SYSENTER_EIP 0x00000176

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