Re: [PATCH v2] x86/microcode/intel: Reject problematic loading on Granite Rapids systems

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/09/2026 11:32 pm, Chang S. Bae wrote:
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 1142183c950c..61ad280497e9 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -309,6 +309,32 @@ static void save_microcode_patch(struct microcode_intel *patch)
>  		pr_err("Unable to allocate microcode memory size: %u\n", size);
>  }
>  
> +static bool revision_banned(struct cpu_signature *sig, u32 rev)
> +{
> +	u32 vfm = IFM(x86_family(sig->sig), x86_model(sig->sig));
> +
> +	/*
> +	 * Revision 0x1000405 contains prerequisite changes for subsequent
> +	 * microcode updates on Granite Rapids systems. Updates directly from
> +	 * an older revision to this or a newer one can result in #MC. This is
> +	 * documented item GNR98, #835486 (Intel Xeon 6900/6700/6500-Series
> +	 * Processors with P-Cores).
> +	 */
> +	if (vfm == INTEL_GRANITERAPIDS_X &&
> +	    x86_stepping(sig->sig) == 1 &&
> +	    sig->pf & 0x95 &&
> +	    sig->rev < 0x1000405 &&
> +	    rev >= 0x1000405) {
> +		if (rev == 0x1000405)
> +			pr_err_once("Erratum GNR98: revision 0x1000405 is not loadable.\n");
> +		else
> +			pr_err_once("Erratum GNR98: revision 0x1000405 is required before 0x%x.\n", rev);
> +		return true;
> +	}

This logic is going to need editing when the muti-blob finally appears.

This is what I'm doing for Xen:
https://lore.kernel.org/xen-devel/20260908171525.3196765-1-andrew.cooper3@citrix.com/T/#u

Notably, 0x1000405 is safe to load if 0x1000380 is in FIT, and this is a
necessary hoop to jump through in due course.

~Andrew