Re: [PATCH v4 04/10] cpu/bugs: Allow spectre_v2=ibrs on x86 vendors other than Intel

[Pawan Gupta]

On Tue, Aug 04, 2026 at 06:56:05PM -0500, Kim Phillips wrote:
...
> @@ -2297,13 +2299,26 @@ static void __init spectre_v2_apply_mitigation(void)
>  	if (spectre_v2_enabled == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
>  		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
>  
> -	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled)) {
> -		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> +	/*
> +	 * head_64.S preserves EFER.AUTOIBRS across boot, so a kexec from a
> +	 * kernel that ran in AutoIBRS mode carries the bit into the new kernel.
> +	 * Explicitly set or clear it to match the selected mitigation, regardless
> +	 * of which mode is in effect.  The boot CPU does this before
> +	 * init_real_mode() snapshots EFER for the AP trampoline, so APs inherit
> +	 * the correct value too.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> +		if (spectre_v2_in_eibrs_mode(spectre_v2_enabled))
>  			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> -		} else {
> -			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> -			update_spec_ctrl(x86_spec_ctrl_base);
> -		}
> +		else
> +			msr_clear_bit(MSR_EFER, _EFER_AUTOIBRS);

Clearing the old kernel state should be done well before the mitigation
selection starts, below already does it for the SPEC_CTRL MSR:

  void __init cpu_select_mitigations(void)
  {
          /*
           * Read the SPEC_CTRL MSR to account for reserved bits which may
           * 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 (cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL)) {
                  rdmsrq(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;
          }

> +	}
> +
> +	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled) &&
> +	    !(boot_cpu_has(X86_FEATURE_AUTOIBRS) &&
> +	      spectre_v2_in_eibrs_mode(spectre_v2_enabled))) {
> +		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> +		update_spec_ctrl(x86_spec_ctrl_base);
>  	}

Nit, I find this a bit confusing. IIUC when AutoIBRS is supported, you want
SPEC_CTRL[IBRS] to be set when legacy IBRS mitigation is deployed. If you
move MSR write down after legacy IBRS mode is set(KERNEL_IBRS), you can do:

          if (spectre_v2_in_ibrs_mode(spectre_v2_enabled)) {
                 if (boot_cpu_has(X86_FEATURE_AUTOIBRS) &&
                     !boot_cpu_has(X86_FEATURE_KERNEL_IBRS)) {    <---- just adding KERNEL_IBRS check to existing code
                          msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
                  } else {
                          x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
                          update_spec_ctrl(x86_spec_ctrl_base);
                  }
          }

Otherwise, a comment explaining the intent would be helpful.