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[PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page

Mingwei Zhang posted 6 patches 2 years, 7 months ago
There is a newer version of this series
[PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page
Posted by Mingwei Zhang 2 years, 7 months ago 
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@google.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
+    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
Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page
Posted by Sean Christopherson 2 years, 7 months ago 
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@google.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.

> +    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
>
Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page
Posted by Mingwei Zhang 2 years, 6 months ago 
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@google.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
> >
Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page
Posted by Huang, Kai 2 years, 7 months ago 
On Mon, 2023-06-26 at 18:20 +0000, 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@google.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
> +    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
> 

Reviewed-by: Kai Huang <kai.huang@intel.com>
Re: [PATCH v2 3/6] KVM: Documentation: Add the missing description for ptep in kvm_mmu_page
Posted by Randy Dunlap 2 years, 7 months ago 

On 6/26/23 11:20, 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@google.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
> +    used in replace of parent_ptes when TDP MMU is used. In TDP MMU, each

            in replacement of

> +    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

-- 
~Randy

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