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

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Date:   Wed, 25 Oct 2023 19:33:31 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, 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,
        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 Mon, Oct 16, 2023 at 08:27:34AM -0500, 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>
> ---
>  arch/x86/kernel/cpu/common.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Acked-by: Borislav Petkov (AMD) <bp@...en8.de>

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette