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

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Date:   Thu, 22 Jun 2023 08:32:03 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mizhang@...gle.com" <mizhang@...gle.com>
CC:     "jmattson@...gle.com" <jmattson@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "bgardon@...gle.com" <bgardon@...gle.com>
Subject: Re: [PATCH 3/6] KVM: Documentation: Add the missing ptep in
 kvm_mmu_page

On Sun, 2023-06-18 at 00:08 +0000, Mingwei Zhang wrote:
> Add the missing 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 | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/x86/mmu.rst b/Documentation/virt/kvm/x86/mmu.rst
> index 149dd3cba48f..36bfe0fe02bb 100644
> --- a/Documentation/virt/kvm/x86/mmu.rst
> +++ b/Documentation/virt/kvm/x86/mmu.rst
> @@ -236,6 +236,10 @@ 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:
> +    Pointer to the parent spte when TDP MMU is enabled. 
> 

IMHO "parent spte" alone _may_ be confusing.  I think it's better to explicitly
mention "pointing to this page" similar to the "parent_ptes" above.

Also, I think "when TDP MMU is enabled" isn't strictly true, depending on what
does "when TDP MMU is enabled mean".  E.g., when tdp_mmu_enabled module
parameter is true, we can still have a nested EPT shadow page from L2 which
won't use this either IIUC.

> In TDP MMU, each
> +    shadow page will have at most one parent. Note that this field is a
> +    union with parent_ptes.

Also, perhaps "have at most one parent" can be more precise: only root page has
no parent, while other non-root pages always have one parent SPTE pointing to
each of them.