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

From: Jim Mattson <jmattson@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] KVM: x86: WARN and reject loading KVM if NX is supported but not enabled
Date: Tue, 15 Jun 2021 15:39:51 -0700	[thread overview]
Message-ID: <CALMp9eSkVaDfCJwW1eds=7H7yn2pKJPKoFVpc1GQcEqGD5S0Dg@mail.gmail.com> (raw)
In-Reply-To: <20210615164535.2146172-4-seanjc@google.com>

On Tue, Jun 15, 2021 at 9:45 AM Sean Christopherson <seanjc@google.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@google.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@google.com>