linux-kernel - Re: [PATCH 4/9] KVM: SVM: Explicitly require DECODEASSISTS to enable SEV support

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Date:   Thu, 20 Jan 2022 14:32:50 +0000
From:   Liam Merwick <liam.merwick@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Liam Merwick <liam.merwick@...cle.com>
Subject: Re: [PATCH 4/9] KVM: SVM: Explicitly require DECODEASSISTS to enable
 SEV support

On 20/01/2022 01:07, Sean Christopherson wrote:
> Add a sanity check on DECODEASSIST being support if SEV is supported, as

"DECODEASSISTS being supported" or "DECODEASSISTS support"

> 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>


> ---
>   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 6a22798eaaee..17b53457d866 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2100,8 +2100,13 @@ 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 */