linux-kernel - Re: [RFC 01/14] KVM: x86/MMU: Add shadow

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Date:   Wed, 1 Feb 2023 11:48:18 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Vipin Sharma <vipinsh@...gle.com>,
        Nagareddy Reddy <nspreddy@...gle.com>
Subject: Re: [RFC 01/14] KVM: x86/MMU: Add shadow_mmu.(c|h)

On Wed, Feb 1, 2023 at 11:45 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Dec 21, 2022, Ben Gardon wrote:
> > diff --git a/arch/x86/kvm/mmu/shadow_mmu.c b/arch/x86/kvm/mmu/shadow_mmu.c
> > new file mode 100644
> > index 000000000000..7bce5ec52b2e
> > --- /dev/null
> > +++ b/arch/x86/kvm/mmu/shadow_mmu.c
> > @@ -0,0 +1,21 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * KVM Shadow MMU
> > + *
> > + * This file implements the Shadow MMU: the KVM MMU implementation which has
> > + * developed organically from hardware which did not have second level paging,
> > + * and so used "shadow paging" to virtualize guest memory. The Shadow MMU is
> > + * an alternative to the TDP MMU which only supports hardware with Two
> > + * Dimentional Paging. (e.g. EPT on Intel or NPT on AMD CPUs.) Note that the
> > + * Shadow MMU also supports TDP, it's just less scalable. The Shadow and TDP
> > + * MMUs can cooperate to support nested virtualization on hardware with TDP.
> > + */
>
> Eh, I vote to omit the comment.  For newbies, Documentation is likely a better
> landing spot for describing the MMUs, and people that are familiar with KVM x86
> MMU already know what the shadow MMU is and does.  That way we avoid bikeshedding
> this comment, at least in the conext of this series.  E.g. I'm pretty sure much
> of the shadow MMU behavior wasn't developed organically, it was stolen from Xen.
> And the line about the Shadow and TDP MMUs cooperating support nested virt is
> loaded with assumptions and qualifiers, and makes it sound like nested virt only
> works with _the_ TDP MMU as oposed to _a_ TDP MMU`.

Sounds good, I can dump the comment. I plan to send out a rebased
version of this series tomorrow, incorporating all the feedback this
series has gotten. Thanks for taking another look at it.