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Subject[PATCH v2 1/8] x86/feature: Detect the x86 IBRS feature to control Speculation
Date
cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature
IA32_SPEC_CTRL (0x48)
IA32_SPEC_CTRL, bit0 – 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 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

* There is no need to clear IBRS before writing it with a value of
1. Unconditionally writing it with a value of 1 after the prediction
mode change is sufficient

* 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@linux.intel.com>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/msr-index.h | 4 ++++
arch/x86/kernel/cpu/scattered.c | 1 +
tools/arch/x86/include/asm/cpufeatures.h | 1 +
4 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 07cdd17..5ee0737 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -209,6 +209,7 @@
#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) /* Control Speculation Control */

/* 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 34c4922..f881add 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_FEATURE_DISABLE_IBRS (0 << 0)
+#define SPEC_CTRL_FEATURE_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 800104c..5e56275 100644
--- a/tools/arch/x86/include/asm/cpufeatures.h
+++ b/tools/arch/x86/include/asm/cpufeatures.h
@@ -208,6 +208,7 @@
#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) /* Control Speculation Control */

/* Virtualization flags: Linux defined, word 8 */
#define X86_FEATURE_TPR_SHADOW ( 8*32+ 0) /* Intel TPR Shadow */
--
2.9.4
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