From: Breno Leitao leitao@debian.org
[ Upstream commit 0125acda7d76b943ca55811df40ed6ec0ecf670f ]
Currently, x86_spec_ctrl_base is read at boot time and speculative bits are set if Kconfig items are enabled. For example, IBRS is enabled if CONFIG_CPU_IBRS_ENTRY is configured, etc. These MSR bits are not cleared if the mitigations are disabled.
This is a problem when kexec-ing a kernel that has the mitigation disabled from a kernel that has the mitigation enabled. In this case, the MSR bits are not cleared during the new kernel boot. As a result, this might have some performance degradation that is hard to pinpoint.
This problem does not happen if the machine is (hard) rebooted because the bit will be cleared by default.
[ bp: Massage. ]
Suggested-by: Pawan Gupta pawan.kumar.gupta@linux.intel.com Signed-off-by: Breno Leitao leitao@debian.org Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Link: https://lore.kernel.org/r/20221128153148.1129350-1-leitao@debian.org Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/include/asm/msr-index.h | 4 ++++ arch/x86/kernel/cpu/bugs.c | 10 +++++++++- 2 files changed, 13 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h index 0bd07699dba38..847f3f5820d21 100644 --- a/arch/x86/include/asm/msr-index.h +++ b/arch/x86/include/asm/msr-index.h @@ -50,6 +50,10 @@ #define SPEC_CTRL_RRSBA_DIS_S_SHIFT 6 /* Disable RRSBA behavior */ #define SPEC_CTRL_RRSBA_DIS_S BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)
+/* A mask for bits which the kernel toggles when controlling mitigations */ +#define SPEC_CTRL_MITIGATIONS_MASK (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD \ + | SPEC_CTRL_RRSBA_DIS_S) + #define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */ #define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index e298ec9d5d536..54f42ae1a61d9 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -135,9 +135,17 @@ void __init check_bugs(void) * have unknown values. AMD64_LS_CFG MSR is cached in the early AMD * init code as it is not enumerated and depends on the family. */ - if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) + if (cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL)) { rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+ /* + * Previously running kernel (kexec), may have some controls + * turned ON. Clear them and let the mitigations setup below + * rediscover them based on configuration. + */ + x86_spec_ctrl_base &= ~SPEC_CTRL_MITIGATIONS_MASK; + } + /* Select the proper CPU mitigations before patching alternatives: */ spectre_v1_select_mitigation(); spectre_v2_select_mitigation();