intel CPUID
The CPUID instruction enumerates support for the mitigation mechanisms using five feature flags in CPUID.(EAX=7H,ECX=0):EDX:
- CPUID.(EAX=7H,ECX=0):EDX[10] indicates the processor is not affected by microarchitectural data sampling (MDS).
- CPUID.(EAX=7H,ECX=0):EDX[26] enumerates support for indirect branch restricted speculation (IBRS) and the indirect branch predictor barrier (IBPB). Processors that set this bit support the IA32_SPEC_CTRL MSR and the IA32_PRED_CMD MSR. They allow software to set IA32_SPEC_CTRL[0] (IBRS) and IA32_PRED_CMD[0] (IBPB).
- CPUID.(EAX=7H,ECX=0):EDX[27] enumerates support for single thread indirect branch predictors (STIBP). Processors that set this bit support the IA32_SPEC_CTRL MSR. They allow software to set IA32_SPEC_CTRL[1] (STIBP).
- CPUID.(EAX=7H,ECX=0):EDX[28] enumerates support for L1D_FLUSH. Processors that set this bit support the IA32_FLUSH_CMD MSR. They allow software to set IA32_FLUSH_CMD[0] (L1D_FLUSH).
- CPUID.(EAX=7H,ECX=0):EDX[29] enumerates support for the IA32_ARCH_CAPABILITIES MSR.
- CPUID.(EAX=7H,ECX=0):EDX[31] enumerates support for Speculative Store Bypass Disable (SSBD). Processors that set this bit support the IA32_SPEC_CTRL MSR. They allow software to set IA32_SPEC_CTRL[2] (SSBD).
The mitigation mechanisms may be introduced to a processor by loading a microcode update. In such cases, software should reevaluate the enumeration after loading that microcode update.
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