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

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

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

Yes, exactly. I meand to say that founding fathers of KVM left the
adjustment for KVM_SET_CPUID exclusively on purpose and not by mistake
:-)

-- 
Vitaly