[PATCH v3 1/5] x86/feature: Detect the x86 IBRS feature to control Speculation

[Tim Chen]

cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature

IA32_SPEC_CTRL (MSR 0x48)
IA32_SPEC_CTRL, bit0 â Indirect Branch Restricted Speculation (IBRS)

If IBRS is set, near returns and near indirect jumps/calls will not allow
their predicted target address to be controlled by code that executed in
a less privileged prediction mode before the IBRS mode was last written
with a value of 1 or on another logical processor so long as all return
stack buffer (RSB) entries from the previous less privileged prediction
mode are overwritten.

* Thus a near indirect jump/call/return may be affected by code in a
less privileged prediction mode that executed AFTER IBRS mode was last
written with a value of 1.

* Note: IBRS is not required in order to isolate branch predictions for
SMM or SGX enclaves.

* Code executed by a sibling logical processor cannot control indirect
jump/call/return predicted target when IBRS is set.

* SMEP will prevent supervisor mode using RSB entries filled by user code;
this can reduce the need for software to overwrite RSB entries.

CPU performance could be reduced when running with IBRS set.

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/cpufeatures.h       | 2 ++
 arch/x86/include/asm/msr-index.h         | 4 ++++
 arch/x86/kernel/cpu/scattered.c          | 1 +
 tools/arch/x86/include/asm/cpufeatures.h | 2 ++
 4 files changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index f275447..624b58e 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -211,6 +211,8 @@
 #define X86_FEATURE_AVX512_4FMAPS	( 7*32+17) /* AVX-512 Multiply Accumulation Single precision */
 
 #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
+#define X86_FEATURE_SPEC_CTRL		( 7*32+19) /* Speculation Control */
+#define X86_FEATURE_SPEC_CTRL_IBRS	( 7*32+20) /* Speculation Control, use IBRS */
 
 /* Virtualization flags: Linux defined, word 8 */
 #define X86_FEATURE_TPR_SHADOW		( 8*32+ 0) /* Intel TPR Shadow */
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index fa11fb1..3e1cb18 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -42,6 +42,10 @@
 #define MSR_PPIN_CTL			0x0000004e
 #define MSR_PPIN			0x0000004f
 
+#define MSR_IA32_SPEC_CTRL		0x00000048
+#define SPEC_CTRL_DISABLE_IBRS		(0 << 0)
+#define SPEC_CTRL_ENABLE_IBRS		(1 << 0)
+
 #define MSR_IA32_PERFCTR0		0x000000c1
 #define MSR_IA32_PERFCTR1		0x000000c2
 #define MSR_FSB_FREQ			0x000000cd
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index 05459ad..bc50c40 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -24,6 +24,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_INTEL_PT,		CPUID_EBX, 25, 0x00000007, 0 },
 	{ X86_FEATURE_AVX512_4VNNIW,    CPUID_EDX,  2, 0x00000007, 0 },
 	{ X86_FEATURE_AVX512_4FMAPS,    CPUID_EDX,  3, 0x00000007, 0 },
+	{ X86_FEATURE_SPEC_CTRL,	CPUID_EDX, 26, 0x00000007, 0 },
 	{ X86_FEATURE_CAT_L3,		CPUID_EBX,  1, 0x00000010, 0 },
 	{ X86_FEATURE_CAT_L2,		CPUID_EBX,  2, 0x00000010, 0 },
 	{ X86_FEATURE_CDP_L3,		CPUID_ECX,  2, 0x00000010, 1 },
diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
index 793690f..995d74b 100644
--- a/tools/arch/x86/include/asm/cpufeatures.h
+++ b/tools/arch/x86/include/asm/cpufeatures.h
@@ -205,6 +205,8 @@
 #define X86_FEATURE_AVX512_4FMAPS (7*32+17) /* AVX-512 Multiply Accumulation Single precision */
 
 #define X86_FEATURE_MBA         ( 7*32+18) /* Memory Bandwidth Allocation */
+#define X86_FEATURE_SPEC_CTRL		( 7*32+19) /* Speculation Control */
+#define X86_FEATURE_SPEC_CTRL_IBRS	( 7*32+20) /* Speculation Control, use IBRS */
 
 /* Virtualization flags: Linux defined, word 8 */
 #define X86_FEATURE_TPR_SHADOW  ( 8*32+ 0) /* Intel TPR Shadow */
-- 
2.9.4