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

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Date:   Fri, 27 Oct 2023 14:50:41 -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,
        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 06:27, Michael Roth wrote:
> 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.

Thanks for the updated changelog:

Acked-by: Dave Hansen <dave.hansen@...el.com>

BTW, have you given your hardware folks a hard time about this?  It
seems _kinda_ silly to be using retpolines when the hardware has a
perfectly good IBRS implementation for the kernel.

Just please make sure there's a good underlying reason for this behavior
and as opposed to being some kind of inadvertent side effect.

I assume Auto-IBRS and SEV-SNP are going to be with us for a long time,
so it would be nice to have a long term solution here.