linux-kernel - Re: [PATCH RFC v9 08/51] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

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Date:   Mon, 12 Jun 2023 08:39:23 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
        rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
        bp@...en8.de, vbabka@...e.cz, kirill@...temov.name,
        ak@...ux.intel.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, liam.merwick@...cle.com,
        zhi.a.wang@...el.com, Kim Phillips <kim.phillips@....com>
Subject: Re: [PATCH RFC v9 08/51] x86/speculation: Do not enable Automatic
 IBRS if SEV SNP is enabled

On 6/11/23 21:25, Michael Roth wrote:
> A hardware limitation prevents the host from enabling Automatic IBRS
> when SNP is enabled.  Instead, fall back to retpolines.

"Hardware limitation"?  As in, it is a documented, architectural
restriction?  Or, it's a CPU bug?

> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index f9d060e71c3e..3fba3623ff64 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1507,7 +1507,12 @@ static void __init spectre_v2_select_mitigation(void)
>  
>  	if (spectre_v2_in_ibrs_mode(mode)) {
>  		if (boot_cpu_has(X86_FEATURE_AUTOIBRS)) {
> -			msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> +			if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP)) {
> +				msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
> +			} else {
> +				pr_err("SNP feature available, not enabling AutoIBRS on the host.\n");
> +				mode = spectre_v2_select_retpoline();
> +			}

I think this would be nicer if you did something like:

	if (cpu_feature_enabled(X86_FEATURE_SEV_SNP))
		setup_clear_cpu_cap(X86_FEATURE_AUTOIBRS);

somewhere _else_ in the code instead of smack-dab in the middle of the
mitigation selection.