linux-kernel - Re: [PATCH 3/4] KVM: x86: WARN and reject loading KVM if NX is supported but not enabled

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Message-ID: <CALMp9eSkVaDfCJwW1eds=7H7yn2pKJPKoFVpc1GQcEqGD5S0Dg@mail.gmail.com>
Date:   Tue, 15 Jun 2021 15:39:51 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] KVM: x86: WARN and reject loading KVM if NX is
 supported but not enabled

On Tue, Jun 15, 2021 at 9:45 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> WARN if NX is reported as supported but not enabled in EFER.  All flavors
> of the kernel, including non-PAE 32-bit kernels, set EFER.NX=1 if NX is
> supported, even if NX usage is disable via kernel command line.  KVM relies
> on NX being enabled if it's supported, e.g. KVM will generate illegal NPT
> entries if nx_huge_pages is enabled and NX is supported but not enabled.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index acc28473dec7..1f6595df45de 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10981,6 +10981,9 @@ 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;

Input/output error? Is that really the most appropriate error here?
Why not, say, -ENOTSUP?

I'm sure there's some arcane convention here that I'm not privy to. :-)

Reviewed-by: Jim Mattson <jmattson@...gle.com>