[PATCH v3 6/7] x86/cpu, kvm: Support AMD Automatic IBRS

Kim Phillips
2022-11-30T00:00:34Z

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.

Also allow for spectre_v2=autoibrs on the kernel command line.
'spectre_v2=autoibrs,retpoline' and 'autoibrs,lfence' are
honoured but not required. AutoIBRS will also be enabled if
the =eibrs[,{lfence,retpoline}] variants are specified.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
.../admin-guide/kernel-parameters.txt | 9 +++++---
arch/x86/include/asm/msr-index.h | 2 ++
arch/x86/kernel/cpu/bugs.c | 23 ++++++++++++-------
arch/x86/kernel/cpu/common.c | 8 +++++++
arch/x86/kvm/svm/svm.c | 3 +++
arch/x86/kvm/x86.c | 3 +++
6 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a465d5242774..880016d06a8a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5698,9 +5698,12 @@
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
+ autoibrs - Enhanced/Auto IBRS
+ eibrs,retpoline - Enhanced/Auto IBRS + Retpolines
+ autoibrs,retpoline- Enhanced/Auto IBRS + Retpolines
+ eibrs,lfence - Enhanced/Auto IBRS + LFENCE
+ autoibrs,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/msr-index.h b/arch/x86/include/asm/msr-index.h
index 8519191c6409..88fdd75f6a2f 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -30,6 +30,7 @@
#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)
@@ -38,6 +39,7 @@
#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 aa0819252c88..5f48dd4dbc48 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1222,9 +1222,9 @@ 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",
};

@@ -1240,8 +1240,11 @@ static const struct {
{ "retpoline,lfence", SPECTRE_V2_CMD_RETPOLINE_LFENCE, false },
{ "retpoline,generic", SPECTRE_V2_CMD_RETPOLINE_GENERIC, false },
{ "eibrs", SPECTRE_V2_CMD_EIBRS, false },
+ { "autoibrs", SPECTRE_V2_CMD_EIBRS, false },
{ "eibrs,lfence", SPECTRE_V2_CMD_EIBRS_LFENCE, false },
+ { "autoibrs,lfence", SPECTRE_V2_CMD_EIBRS_LFENCE, false },
{ "eibrs,retpoline", SPECTRE_V2_CMD_EIBRS_RETPOLINE, false },
+ { "autoibrs,retpoline", SPECTRE_V2_CMD_EIBRS_RETPOLINE, false },
{ "auto", SPECTRE_V2_CMD_AUTO, false },
{ "ibrs", SPECTRE_V2_CMD_IBRS, false },
};
@@ -1293,7 +1296,7 @@ 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;
}
@@ -1479,8 +1482,12 @@ 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;
- write_spec_ctrl_current(x86_spec_ctrl_base, true);
+ if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
+ msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+ } else {
+ x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
+ write_spec_ctrl_current(x86_spec_ctrl_base, true);
+ }
}

switch (mode) {
@@ -1564,8 +1571,8 @@ 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 8d28cd7c9072..0af7b963f2e4 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1406,6 +1406,14 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
!(ia32_cap & ARCH_CAP_PBRSB_NO))
setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB);

+ /*
+ * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel flag only
+ * after IBRS_ENHANCED bugs such as BUG_EIBRS_PBRSB above have been
+ * determined.
+ */
+ if (cpu_has(c, X86_FEATURE_AUTOIBRS))
+ setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
+
if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
return;

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4b6d2b050e57..3ac3d4cfce24 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4960,6 +4960,9 @@ static __init int svm_hardware_setup(void)

tsc_aux_uret_slot = kvm_add_user_return_msr(MSR_TSC_AUX);

+ if (boot_cpu_has(X86_FEATURE_AUTOIBRS))
+ kvm_enable_efer_bits(EFER_AUTOIBRS);
+
/* Check for pause filtering support */
if (!boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
pause_filter_count = 0;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 490ec23c8450..db0f522fd597 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1682,6 +1682,9 @@ static int do_get_msr_feature(struct kvm_vcpu *vcpu, unsigned index, u64 *data)

static bool __kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
{
+ if (efer & EFER_AUTOIBRS && !guest_cpuid_has(vcpu, X86_FEATURE_AUTOIBRS))
+ return false;
+
if (efer & EFER_FFXSR && !guest_cpuid_has(vcpu, X86_FEATURE_FXSR_OPT))
return false;

--
2.34.1