linux-kernel - Re: [PATCH] KVM: x86: filter guest NX capability for cpuid2

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Message-ID: <87ft6s8zdg.fsf@vitty.brq.redhat.com>
Date:   Mon, 05 Oct 2020 17:29:47 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: filter guest NX capability for cpuid2

Tianjia Zhang <tianjia.zhang@...ux.alibaba.com> writes:

> Original KVM_SET_CPUID has removed NX on non-NX hosts as it did
> before. but KVM_SET_CPUID2 does not. The two should be consistent.
>
> Signed-off-by: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
> ---
>  arch/x86/kvm/cpuid.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 3fd6eec202d7..3e7ba2b11acb 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -257,6 +257,7 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
>  		goto out;
>  	}
>  
> +	cpuid_fix_nx_cap(vcpu);
>  	kvm_update_cpuid_runtime(vcpu);
>  	kvm_vcpu_after_set_cpuid(vcpu);
>  out:

I stumbled upon this too and came to the conclusion this is
intentional, e.g. see this:

commit 0771671749b59a507b6da4efb931c44d9691e248
Author: Dan Kenigsberg <danken@...ranet.com>
Date:   Wed Nov 21 17:10:04 2007 +0200

    KVM: Enhance guest cpuid management

...

    [avi: fix original KVM_SET_CPUID not removing nx on non-nx hosts as it did
          before]

but this is a very, very old story.

-- 
Vitaly