[PATCH v7 6/7] x86/cpu: Support AMD Automatic IBRS

Kim Phillips
2023-01-16T23:04:50Z

The AMD Zen4 core supports a new feature called Automatic IBRS.

It is a "set-and-forget" feature that means that, like
Intel's Enhanced IBRS, h/w manages its IBRS mitigation
resources automatically across CPL transitions.

The feature is advertised by CPUID_Fn80000021_EAX bit 8 and is
enabled by setting MSR C000_0080 (EFER) bit 21.

Enable Automatic IBRS by default if the CPU feature is present.
It typically provides greater performance over the incumbent
generic retpolines mitigation.

Reuse the SPECTRE_V2_EIBRS spectre_v2_mitigation enum.
AMD Automatic IBRS and Intel Enhanced IBRS have similar
bugs.c enablement. Add NO_EIBRS_PBRSB to cpu_vuln_whitelist,
since AMD Automatic IBRS isn't affected by PBRSB-eIBRS.

The kernel command line option spectre_v2=eibrs is
used to select AMD Automatic IBRS, if available.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
---
Documentation/admin-guide/hw-vuln/spectre.rst | 6 +++---
.../admin-guide/kernel-parameters.txt | 6 +++---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/msr-index.h | 2 ++
arch/x86/kernel/cpu/bugs.c | 20 +++++++++++--------
arch/x86/kernel/cpu/common.c | 19 ++++++++++--------
6 files changed, 32 insertions(+), 22 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index c4dcdb3d0d45..3fe6511c5405 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -610,9 +610,9 @@ kernel command line.
retpoline,generic Retpolines
retpoline,lfence LFENCE; indirect branch
retpoline,amd alias for retpoline,lfence
- eibrs enhanced IBRS
- eibrs,retpoline enhanced IBRS + Retpolines
- eibrs,lfence enhanced IBRS + LFENCE
+ eibrs Enhanced/Auto IBRS
+ eibrs,retpoline Enhanced/Auto IBRS + Retpolines
+ eibrs,lfence Enhanced/Auto IBRS + LFENCE
ibrs use IBRS to protect kernel

Not specifying this option is equivalent to
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf..839fa0fefb58 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5729,9 +5729,9 @@
retpoline,generic - Retpolines
retpoline,lfence - LFENCE; indirect branch
retpoline,amd - alias for retpoline,lfence
- eibrs - enhanced IBRS
- eibrs,retpoline - enhanced IBRS + Retpolines
- eibrs,lfence - enhanced IBRS + LFENCE
+ eibrs - Enhanced/Auto IBRS
+ eibrs,retpoline - Enhanced/Auto IBRS + Retpolines
+ eibrs,lfence - Enhanced/Auto IBRS + LFENCE
ibrs - use IBRS to protect kernel

Not specifying this option is equivalent to
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 861d312c7955..d5acc4dc5906 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -432,6 +432,7 @@
#define X86_FEATURE_NO_NESTED_DATA_BP (20*32+ 0) /* "" AMD No Nested Data Breakpoints */
#define X86_FEATURE_LFENCE_RDTSC (20*32+ 2) /* "" LFENCE always serializing / synchronizes RDTSC */
#define X86_FEATURE_NULL_SEL_CLR_BASE (20*32+ 6) /* "" AMD Null Selector Clears Base */
+#define X86_FEATURE_AUTOIBRS (20*32+ 8) /* "" AMD Automatic IBRS */
#define X86_FEATURE_NO_SMM_CTL_MSR (20*32+ 9) /* "" AMD SMM_CTL MSR is not present */

/*
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index a85019756003..cb3d0f6e6ac2 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -25,6 +25,7 @@
#define _EFER_SVME 12 /* Enable virtualization */
#define _EFER_LMSLE 13 /* Long Mode Segment Limit Enable */
#define _EFER_FFXSR 14 /* Enable Fast FXSAVE/FXRSTOR */
+#define _EFER_AUTOIBRS 21 /* Enable Automatic IBRS */

#define EFER_SCE (1<<_EFER_SCE)
#define EFER_LME (1<<_EFER_LME)
@@ -33,6 +34,7 @@
#define EFER_SVME (1<<_EFER_SVME)
#define EFER_LMSLE (1<<_EFER_LMSLE)
#define EFER_FFXSR (1<<_EFER_FFXSR)
+#define EFER_AUTOIBRS (1<<_EFER_AUTOIBRS)

/* Intel MSRs. Some also available on other CPUs */

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4a0add86c182..cf81848b72f4 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1238,9 +1238,9 @@ static const char * const spectre_v2_strings[] = {
[SPECTRE_V2_NONE] = "Vulnerable",
[SPECTRE_V2_RETPOLINE] = "Mitigation: Retpolines",
[SPECTRE_V2_LFENCE] = "Mitigation: LFENCE",
- [SPECTRE_V2_EIBRS] = "Mitigation: Enhanced IBRS",
- [SPECTRE_V2_EIBRS_LFENCE] = "Mitigation: Enhanced IBRS + LFENCE",
- [SPECTRE_V2_EIBRS_RETPOLINE] = "Mitigation: Enhanced IBRS + Retpolines",
+ [SPECTRE_V2_EIBRS] = "Mitigation: Enhanced / Automatic IBRS",
+ [SPECTRE_V2_EIBRS_LFENCE] = "Mitigation: Enhanced / Automatic IBRS + LFENCE",
+ [SPECTRE_V2_EIBRS_RETPOLINE] = "Mitigation: Enhanced / Automatic IBRS + Retpolines",
[SPECTRE_V2_IBRS] = "Mitigation: IBRS",
};

@@ -1309,7 +1309,7 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
cmd == SPECTRE_V2_CMD_EIBRS_LFENCE ||
cmd == SPECTRE_V2_CMD_EIBRS_RETPOLINE) &&
!boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
- pr_err("%s selected but CPU doesn't have eIBRS. Switching to AUTO select\n",
+ pr_err("%s selected but CPU doesn't have Enhanced or Automatic IBRS. Switching to AUTO select\n",
mitigation_options[i].option);
return SPECTRE_V2_CMD_AUTO;
}
@@ -1495,8 +1495,12 @@ static void __init spectre_v2_select_mitigation(void)
pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);

if (spectre_v2_in_ibrs_mode(mode)) {
- x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
- update_spec_ctrl(x86_spec_ctrl_base);
+ if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+ msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+ } else {
+ x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+ update_spec_ctrl(x86_spec_ctrl_base);
+ }
}

switch (mode) {
@@ -1580,8 +1584,8 @@ static void __init spectre_v2_select_mitigation(void)
/*
* Retpoline protects the kernel, but doesn't protect firmware. IBRS
* and Enhanced IBRS protect firmware too, so enable IBRS around
- * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
- * enabled.
+ * firmware calls only when IBRS / Enhanced / Automatic IBRS aren't
+ * otherwise enabled.
*
* Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
* the user might select retpoline on the kernel command line and if
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index d762654d16a0..b441758d2680 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1229,8 +1229,8 @@ static const __initconst struct x86_cpu_id cpu_vuln_whitelist[] = {
VULNWL_AMD(0x12, NO_MELTDOWN | NO_SSB | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),

/* FAMILY_ANY must be last, otherwise 0x0f - 0x12 matches won't work */
- VULNWL_AMD(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
- VULNWL_HYGON(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO),
+ VULNWL_AMD(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB),
+ VULNWL_HYGON(X86_FAMILY_ANY, NO_MELTDOWN | NO_L1TF | NO_MDS | NO_SWAPGS | NO_ITLB_MULTIHIT | NO_MMIO | NO_EIBRS_PBRSB),

/* Zhaoxin Family 7 */
VULNWL(CENTAUR, 7, X86_MODEL_ANY, NO_SPECTRE_V2 | NO_SWAPGS | NO_MMIO),
@@ -1341,8 +1341,16 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
!cpu_has(c, X86_FEATURE_AMD_SSB_NO))
setup_force_cpu_bug(X86_BUG_SPEC_STORE_BYPASS);

- if (ia32_cap & ARCH_CAP_IBRS_ALL)
+ /*
+ * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel feature
+ * flag and protect from vendor-specific bugs via the whitelist.
+ */
+ if ((ia32_cap & ARCH_CAP_IBRS_ALL) || cpu_has(c, X86_FEATURE_AUTOIBRS)) {
setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
+ if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
+ !(ia32_cap & ARCH_CAP_PBRSB_NO))
+ setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB);
+ }

if (!cpu_matches(cpu_vuln_whitelist, NO_MDS) &&
!(ia32_cap & ARCH_CAP_MDS_NO)) {
@@ -1404,11 +1412,6 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
setup_force_cpu_bug(X86_BUG_RETBLEED);
}

- if (cpu_has(c, X86_FEATURE_IBRS_ENHANCED) &&
- !cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
- !(ia32_cap & ARCH_CAP_PBRSB_NO))
- setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB);
-
if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
return;

--
2.34.1