linux-kernel - Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu

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Date:   Mon, 31 Jul 2023 18:09:40 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kai Huang <kai.huang@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>, Xu Yilun <yilun.xu@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>
Subject: Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description
 for ptep in kvm_mmu_page

On Tue, Jun 27, 2023, Sean Christopherson wrote:
> On Mon, Jun 26, 2023, Mingwei Zhang wrote:
> > Add the missing description for ptep in kvm_mmu_page description. ptep is
> > used when TDP MMU is enabled and it shares the storage with parent_ptes.
> > Update the doc to help readers to get up-to-date info.
> > 
> > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > ---
> >  Documentation/virt/kvm/x86/mmu.rst | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst
> > index 4c9044b4dc6c..5cd6cd5e8926 100644
> > --- a/Documentation/virt/kvm/x86/mmu.rst
> > +++ b/Documentation/virt/kvm/x86/mmu.rst
> > @@ -237,6 +237,11 @@ Shadow pages contain the following information:
> >      parent_ptes points at this single spte, otherwise, there exists multiple
> >      sptes pointing at this page and (parent_ptes & ~0x1) points at a data
> >      structure with a list of parent sptes.
> > +  ptep:
> > +    The reverse mapping for the pte pointing at this page's spt. This field is
> 
> I don't think describing "reverse mapping" is necessary, and it's arguably even
> misleading.  A "reverse mapping" typically provides a way to find mappings given
> a (guest) physical address.  The TDP MMU doesn't bother with reverse mappings
> because there is exactly one possible mapping for any given gfn.  The "ptep" exists
> specifically to expedite zapping a single TDP MMU shadow page, i.e. allows zapping
> without having to traverse the paging tree.
> 
> The ptep field is just a pointer at the SPTE, no more no less.  Something like
> this?
> 
>   ptep:
>     The kernel virtual address of the SPTE that points at this shadow page.
>     Used exclusively by the TDP MMU, this field is a union with parent_ptes.
> 

Sure, I can this version instead. Technically, it is still a reverse
mapping, but yeah, I agree that introducing this one is confusing.

> > +    used in replace of parent_ptes when TDP MMU is used. In TDP MMU, each
> > +    non-root shadow page will have one parent, while each root shadow page has
> > +    no parent. Note that this field is a union with parent_ptes.
> >    unsync:
> >      If true, then the translations in this page may not match the guest's
> >      translation.  This is equivalent to the state of the tlb when a pte is
> > -- 
> > 2.41.0.162.gfafddb0af9-goog
> >