Re: [PATCH v4 02/10] cpu/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs

[Kim Phillips]

On 8/18/26 7:12 PM, Pawan Gupta wrote:
> On Tue, Aug 04, 2026 at 06:56:03PM -0500, Kim Phillips wrote:
> ...
> > @@ -2192,7 +2193,12 @@ static void __init spectre_v2_select_mitigation(void)
> >   			break;
> >   		fallthrough;
> >   	case SPECTRE_V2_CMD_FORCE:
> > -		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
> > +		/*
> > +		 * Don't use AutoIBRS when SNP is enabled because it degrades
> > +		 * host userspace indirect branch performance.
> > +		 */
> > +		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) &&
> > +		    !boot_cpu_has(X86_FEATURE_SEV_SNP)) {
> >   			spectre_v2_enabled = SPECTRE_V2_EIBRS;
> >   			break;
> >   		}
> Can we also account for retpoline availability here? If both SNP and
> retpoline are available, then only avoid AutoIBRS:
>
>            case SPECTRE_V2_CMD_FORCE:
>                    /*
>                     * Prefer retpoline when SNP is enabled because Auto-IBRS
>                     * degrades host userspace indirect branch performance.
>                     */
>                    if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) &&
>                       !(boot_cpu_has(X86_FEATURE_SEV_SNP) &&
>                         IS_ENABLED(CONFIG_MITIGATION_RETPOLINE))) {
>                            spectre_v2_enabled = SPECTRE_V2_EIBRS;
>                            break;
>                    }
>
> With this, patch 1 & 3 can be omitted. Also avoids the ping pong: we don't
> want AutoIBRS -> We don't have retpoline -> Lets have AutoIBRS.
Thank you for your constructive review - I have addressed this comment 
in v5:

https://lore.kernel.org/lkml/20260826223510.3669875-1-kim.phillips@xxxxxxx/

Thanks,

Kim