linux-kernel - Re: [PATCH v3 4/6] KVM: Documentation: Add the missing description for tdp_mmu_root_count into kvm_mmu

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Message-ID: <ZQCswf8EWAGy8QZI@google.com>
Date:   Tue, 12 Sep 2023 18:24:01 +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>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v3 4/6] KVM: Documentation: Add the missing description
 for tdp_mmu_root_count into kvm_mmu_page

On Wed, Aug 16, 2023, Sean Christopherson wrote:
> On Tue, Aug 01, 2023, Mingwei Zhang wrote:
> > Add the description of tdp_mmu_root_count into kvm_mmu_page description and
> > combine it with the description of root_count. tdp_mmu_root_count is an
> > atomic counter used only in TDP MMU. Update the doc.
> > 
> > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > Reviewed-by: Kai Huang <kai.huang@...el.com>
> > ---
> >  Documentation/virt/kvm/x86/mmu.rst | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst
> > index 17d90974204e..40daf8beb9b1 100644
> > --- a/Documentation/virt/kvm/x86/mmu.rst
> > +++ b/Documentation/virt/kvm/x86/mmu.rst
> > @@ -229,10 +229,14 @@ Shadow pages contain the following information:
> >      can be calculated from the gfn field when used.  In addition, when
> >      role.direct is set, KVM does not track access permission for each of the
> >      gfn. See role.direct and gfn.
> > -  root_count:
> > -    A counter keeping track of how many hardware registers (guest cr3 or
> > -    pdptrs) are now pointing at the page.  While this counter is nonzero, the
> > -    page cannot be destroyed.  See role.invalid.
> > +  root_count / tdp_mmu_root_count:
> > +     root_count is a reference counter for root shadow pages in Shadow MMU.
> > +     vCPUs elevate the refcount when getting a shadow page that will be used as
> > +     a root page, i.e. page that will be loaded into hardware directly (CR3,
> > +     PDPTRs, nCR3 EPTP). Root pages cannot be destroyed while their refcount is
> > +     non-zero. See role.invalid. tdp_mmu_root_count is similar but exclusively
> > +     used in TDP MMU as an atomic refcount. When the value is non-zero, it
> > +     allows vCPUs acquire references while holding mmu_lock for read.
> 
> That last sentence is wrong.  *vCPUs* can't acquire references while holding
> mmu_lock for read.  And actually, they don't ever put references while holding
> for read either.  vCPUs *must* hold mmu_lock for write to obtain a new root,
> Not putting references while holding mmu_lock for read is mostly an implementation
> quirk.
> 
> Maybe replace it with this?
> 
>     tdp_mmu_root_count is similar but exclusively used in the TDP MMU as an
>     atomic refcount (select TDP MMU flows walk all roots while holding mmu_lock
>     for read, e.g. when clearing dirty bits).

hmm, I think all the content within the bracket is details and we should
not mention them at all. In fact, when I see the implementation, the
last refcount of tdp_mmu_root_count is treated differently. Those
details should be instead mentioned in code or comments instead of
documentation as they may evolve much faster.

So, I will remove the last sentence.