linux-kernel - Re: [PATCH v10 05/50] x86/speculation: Do not enable Automatic IBRS if SEV SNP is enabled

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Message-ID: <addcbff6-5db9-446a-a0a2-78f3be7f33db@redhat.com>
Date:   Wed, 13 Dec 2023 13:52:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.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, 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,
        jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com,
        pankaj.gupta@....com, liam.merwick@...cle.com,
        zhi.a.wang@...el.com, Kim Phillips <kim.phillips@....com>
Subject: Re: [PATCH v10 05/50] x86/speculation: Do not enable Automatic IBRS
 if SEV SNP is enabled

On 10/16/23 15:27, Michael Roth wrote:
> From: Kim Phillips <kim.phillips@....com>
> 
> Without SEV-SNP, Automatic IBRS protects only the kernel. But when
> SEV-SNP is enabled, the Automatic IBRS protection umbrella widens to all
> host-side code, including userspace. This protection comes at a cost:
> reduced userspace indirect branch performance.
> 
> To avoid this performance loss, don't use Automatic IBRS on SEV-SNP
> hosts. Fall back to retpolines instead.
> 
> Signed-off-by: Kim Phillips <kim.phillips@....com>
> [mdr: squash in changes from review discussion]
> Signed-off-by: Michael Roth <michael.roth@....com>

Queued, thanks.

Paolo

> ---
>   arch/x86/kernel/cpu/common.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 382d4e6b848d..11fae89b799e 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1357,8 +1357,13 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
>   	/*
>   	 * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel feature
>   	 * flag and protect from vendor-specific bugs via the whitelist.
> +	 *
> +	 * Don't use AutoIBRS when SNP is enabled because it degrades host
> +	 * userspace indirect branch performance.
>   	 */
> -	if ((ia32_cap & ARCH_CAP_IBRS_ALL) || cpu_has(c, X86_FEATURE_AUTOIBRS)) {
> +	if ((ia32_cap & ARCH_CAP_IBRS_ALL) ||
> +	    (cpu_has(c, X86_FEATURE_AUTOIBRS) &&
> +	     !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {
>   		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
>   		if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
>   		    !(ia32_cap & ARCH_CAP_PBRSB_NO))