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

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Date:   Wed, 13 Apr 2022 14:42:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        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@...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, Paolo Bonzini wrote:
> On 4/12/22 23:31, Sean Christopherson wrote:
> > > +		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;
> > 
> > Where the field is ignored if direct=1, '0' for non-passthrough, and 1-3 to handle
> > shadow_root_level - guest_root_level.  Basically the same idea as Paolo's smushing
> > of direct+passthrough into mapping_level, just dressed up differently.
> 
> Basically, your passthrough_delta is level - glevel in Jiangshan's patches.
> You'll need 3 bits anyway when we remove direct later (that would be
> passthrough_delta == level).

Are we planning on removing direct?

> Regarding the naming:
> 
> * If we keep Jiangshan's logic, I don't like the glevel name very much, any
> of mapping_level, target_level or direct_level would be clearer?

I don't love any of these names, especially glevel, because the field doesn't
strictly track the guest/mapping/target/direct level.  That could obviously be
remedied by making it valid at all times, but then the role would truly need 3
bits (on top of direct) to track 5-level guest paging.

> * If we go with yours, I would call the field "passthrough_levels".

Hmm, it's not a raw level though.