linux-kernel - Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable

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Message-ID: <CAJhGHyBNK=t1M2QQBq2qea4gW3yqvqKNG0QX=HRvjM=eHY6w6w@mail.gmail.com>
Date:   Wed, 13 Apr 2022 12:13:27 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.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>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level
 expanded pagetable

On Wed, Apr 13, 2022 at 5:31 AM Sean Christopherson <seanjc@...gle.com> wrote:

> >  union kvm_mmu_page_role {
> >       u32 word;
> > @@ -331,7 +331,8 @@ union kvm_mmu_page_role {
> >               unsigned smap_andnot_wp:1;
> >               unsigned ad_disabled:1;
> >               unsigned guest_mode:1;
> > -             unsigned :6;
> > +             unsigned glevel:4;
>
> We don't need 4 bits for this.  Crossing our fingers that we never had to shadow
> a 2-level guest with a 6-level host, we can do:
>
>                 unsigned passthrough_delta:2;

We can save the bits in the future when we need more bits so I didn't
hesitate to use 4 bits since glevel gives simple code. ^_^

I think the name passthrough_delta is more informative and glevel
is used only for comparison so passthrough_delta can also be
simple.  I will apply your suggestion.


Thanks
Lai