Re: [RFC PATCH] KVM: SVM: Disallow SEV if NPT is disabled

[Brijesh Singh]

On 7/31/20 3:51 PM, Sean Christopherson wrote:
> Forcefully turn off SEV if NPT is disabled, e.g. via module param.  SEV
> requires NPT as the C-bit only exists if NPT is active.
>
> Fixes: e9df09428996f ("KVM: SVM: Add sev module_param")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>
> RFC as it's entirely possible that I am completely misunderstanding how
> SEV works.  Compile tested only.


Reviewed-By: Brijesh Singh <brijesh.singh@xxxxxxx>


>
>  arch/x86/kvm/svm/svm.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 783330d0e7b88..e30629593458b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -860,8 +860,14 @@ static __init int svm_hardware_setup(void)
>  		kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
>  	}
>  
> +	if (!boot_cpu_has(X86_FEATURE_NPT))
> +		npt_enabled = false;
> +
> +	if (npt_enabled && !npt)
> +		npt_enabled = false;
> +
>  	if (sev) {
> -		if (boot_cpu_has(X86_FEATURE_SEV) &&
> +		if (boot_cpu_has(X86_FEATURE_SEV) && npt_enabled &&
>  		    IS_ENABLED(CONFIG_KVM_AMD_SEV)) {
>  			r = sev_hardware_setup();
>  			if (r)
> @@ -879,12 +885,6 @@ static __init int svm_hardware_setup(void)
>  			goto err;
>  	}
>  
> -	if (!boot_cpu_has(X86_FEATURE_NPT))
> -		npt_enabled = false;
> -
> -	if (npt_enabled && !npt)
> -		npt_enabled = false;
> -
>  	kvm_configure_mmu(npt_enabled, PG_LEVEL_1G);
>  	pr_info("kvm: Nested Paging %sabled\n", npt_enabled ? "en" : "dis");
>