linux-kernel - Re: [RFC PATCH 4/6] KVM: X86: Introduce role.level

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Date:   Fri, 11 Feb 2022 17:06:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        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>
Subject: Re: [RFC PATCH 4/6] KVM: X86: Introduce role.level_promoted

On 1/4/22 23:14, Sean Christopherson wrote:
> Alternatively, should we mark passthrough shadow pages as direct=1?  That would
> naturally handle this code, and for things like reexecute_instruction()'s usage
> of kvm_mmu_unprotect_page(), I don't think passthrough shadow pages should be
> considered indirect, e.g. zapping them won't help and the shadow page can't become
> unsync.

So the main difference between direct and passthrough shadow pages is that
passthrough pages can have indirect children.  A direct page maps the
page at sp->gfn, while a passthrough page maps the page _table_ at
sp->gfn.

Is this correct?

If so, I think there is a difference between a passthrough page that
maps a level-2 page from level-4, and a passthrough page that maps a
level-3 page from level-4.  This means that a single bit in the role
is not enough.

One way to handle this could be to have a single field "direct_levels"
that subsumes both "direct" and "passthrough".  direct && !passthrough
would correspond to "direct_levels == level", while !direct && !passthrough
would correspond to "direct_levels == 0".

Paolo