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

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Oct 05, 2020 at 05:29:47PM +0200, Vitaly Kuznetsov wrote:
> Tianjia Zhang <tianjia.zhang@linux.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@linux.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@qumranet.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.

Doesn't mean it's bogus though :-)  _If_ we want to extend this behavior to
KVM_SET_CPUID2, there should be a justified need.