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

From: Lai Jiangshan <jiangshanlai@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Lai Jiangshan <jiangshan.ljs@antgroup.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	linux-doc@vger.kernel.org
Subject: Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable
Date: Wed, 13 Apr 2022 12:13:27 +0800	[thread overview]
Message-ID: <CAJhGHyBNK=t1M2QQBq2qea4gW3yqvqKNG0QX=HRvjM=eHY6w6w@mail.gmail.com> (raw)
In-Reply-To: <YlXvtMqWpyM9Bjox@google.com>

On Wed, Apr 13, 2022 at 5:31 AM Sean Christopherson <seanjc@google.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