Re: [PATCH v7 044/102] KVM: x86/mmu: Add a private pointer to struct kvm_mmu_page

Kai Huang
2022-07-01T11:12:53Z

On Mon, 2022-06-27 at 14:53 -0700, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> For private GPA, CPU refers a private page table whose contents are
> encrypted. The dedicated APIs to operate on it (e.g. updating/reading its
> PTE entry) are used and their cost is expensive.
>
> When KVM resolves KVM page fault, it walks the page tables. To reuse the
> existing KVM MMU code and mitigate the heavy cost to directly walk
> encrypted private page table, allocate a more page to mirror the existing
> KVM page table.  Resolve KVM page fault with the existing code, and do
> additional operations necessary for the mirrored private page table. To
> distinguish such cases, the existing KVM page table is called a shared page
> table (i.e. no mirrored private page table), and the KVM page table with
> mirrored private page table is called a private page table. The
> relationship is depicted below.
>
> Add private pointer to struct kvm_mmu_page for mirrored private page table
> and add helper functions to allocate/initialize/free a mirrored private
> page table page. Also, add helper functions to check if a given
> kvm_mmu_page is private. The later patch introduces hooks to operate on
> the mirrored private page table.
>
> KVM page fault |
> | |
> V |
> -------------+---------- |
> | | |
> V V |
> shared GPA private GPA |
> | | |
> V V |
> CPU/KVM shared PT root KVM private PT root | CPU private PT root
> | | | |
> V V | V
> shared PT private PT <----mirror----> mirrored private PT
> | | | |
> | \-----------------+------\ |
> | | | |
> V | V V
> shared guest page | private guest page
> |
> non-encrypted memory | encrypted memory
> |
> PT: page table
>
> Both CPU and KVM refer to CPU/KVM shared page table. Private page table
> is used only by KVM. CPU refers to mirrored private page table.

Shouldn't the private page table maintained by KVM be "mirrored private PT"?

To me "mirrored" normally implies it is fake, or backup which isn't actually
used. But here "mirrored private PT" is actually used by hardware.

And to me, "CPU and KVM" above are confusing. For instance, "Both CPU and KVM
refer to CPU/KVM shared page table" took me at least one minute to understand,
with the help from the diagram -- otherwise I won't be able to understand.

I guess you can just say somewhere:

1) Shared PT is visible to KVM and it is used by CPU;
1) Private PT is used by CPU but it is invisible to KVM;
2) Mirrored private PT is visible to KVM but not used by CPU. It is used to
mirror the actual private PT which is used by CPU.

[...]

> +
> +static inline void kvm_mmu_init_private_sp(struct kvm_mmu_page *sp, void *private_sp)
> +{
> + sp->private_sp = private_sp;
> +}
>

[...]

> @@ -295,6 +297,7 @@ static void tdp_mmu_init_sp(struct kvm_mmu_page *sp, tdp_ptep_t sptep,
> sp->gfn = gfn;
> sp->ptep = sptep;
> sp->tdp_mmu_page = true;
> + kvm_mmu_init_private_sp(sp);

Can this even compile? Unless I am seeing mistakenly, kvm_mmu_init_private_sp()
(see above) has two arguments..

Please make sure each patch can at least compile and doesn't cause warning...

--
Thanks,
-Kai