linux-kernel - Re: [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is supported but not enabled"

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Date:   Thu, 8 Jul 2021 18:17:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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
Subject: Re: [PATCH] Revert "KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled"

On 25/06/21 02:18, Sean Christopherson wrote:
> Let KVM load if EFER.NX=0 even if NX is supported, the analysis and
> testing (or lack thereof) for the non-PAE host case was garbage.
> 
> If the kernel won't be using PAE paging, .Ldefault_entry in head_32.S
> skips over the entire EFER sequence.  Hopefully that can be changed in
> the future to allow KVM to require EFER.NX, but the motivation behind
> KVM's requirement isn't yet merged.  Reverting and revisiting the mess
> at a later date is by far the safest approach.
> 
> This reverts commit 8bbed95d2cb6e5de8a342d761a89b0a04faed7be.
> 
> Fixes: 8bbed95d2cb6 ("KVM: x86: WARN and reject loading KVM if NX is supported but not enabled")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> 
> Hopefully it's not too late to just drop the original patch...
> 
>   arch/x86/kvm/x86.c | 3 ---
>   1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4a597aafe637..1cc02a3685d0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10981,9 +10981,6 @@ int kvm_arch_hardware_setup(void *opaque)
>   	int r;
>   
>   	rdmsrl_safe(MSR_EFER, &host_efer);
> -	if (WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_NX) &&
> -			 !(host_efer & EFER_NX)))
> -		return -EIO;
>   
>   	if (boot_cpu_has(X86_FEATURE_XSAVES))
>   		rdmsrl(MSR_IA32_XSS, host_xss);
> 

So do we want this or "depends on X86_64 || X86_PAE"?

Paolo