linux-kernel - Re: [PATCH MANUALSEL 5.16 6/8] KVM: SVM: Explicitly require DECODEASSISTS to enable SEV support

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Date:   Thu, 10 Feb 2022 17:33:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Liam Merwick <liam.merwick@...cle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.16 6/8] KVM: SVM: Explicitly require
 DECODEASSISTS to enable SEV support

On 2/9/22 19:56, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> [ Upstream commit c532f2903b69b775d27016511fbe29a14a098f95 ]
> 
> Add a sanity check on DECODEASSIST being support if SEV is supported, as
> KVM cannot read guest private memory and thus relies on the CPU to
> provide the instruction byte stream on #NPF for emulation.  The intent of
> the check is to document the dependency, it should never fail in practice
> as producing hardware that supports SEV but not DECODEASSISTS would be
> non-sensical.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Liam Merwick <liam.merwick@...cle.com>
> Message-Id: <20220120010719.711476-5-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/svm/sev.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index be28831412209..932afd713a02c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2099,8 +2099,13 @@ void __init sev_hardware_setup(void)
>   	if (!sev_enabled || !npt_enabled)
>   		goto out;
>   
> -	/* Does the CPU support SEV? */
> -	if (!boot_cpu_has(X86_FEATURE_SEV))
> +	/*
> +	 * SEV must obviously be supported in hardware.  Sanity check that the
> +	 * CPU supports decode assists, which is mandatory for SEV guests to
> +	 * support instruction emulation.
> +	 */
> +	if (!boot_cpu_has(X86_FEATURE_SEV) ||
> +	    WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_DECODEASSISTS)))
>   		goto out;
>   
>   	/* Retrieve SEV CPUID information */

NACK