Re: [PATCH Part2 v6 31/49] KVM: x86: Introduce kvm_mmu_get_tdp_walk() for SEV-SNP use

[Alper Gun]

On Mon, Jun 20, 2022 at 4:09 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
>
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> The SEV-SNP VMs may call the page state change VMGEXIT to add the GPA
> as private or shared in the RMP table. The page state change VMGEXIT
> will contain the RMP page level to be used in the RMP entry. If the
> page level between the TDP and RMP does not match then, it will result
> in nested-page-fault (RMP violation).
>
> The SEV-SNP VMGEXIT handler will use the kvm_mmu_get_tdp_walk() to get
> the current page-level in the TDP for the given GPA and calculate a
> workable page level. If a GPA is mapped as a 4K-page in the TDP, but
> the guest requested to add the GPA as a 2M in the RMP entry then the
> 2M request will be broken into 4K-pages to keep the RMP and TDP
> page-levels in sync.
>
> TDP SPTEs are RCU protected so need to put kvm_mmu_get_tdp_walk() in RCU
> read-side critical section by using walk_shadow_page_lockless_begin() and
> walk_lockless_shadow_page_lockless_end(). This fixes the
> "suspicious RCU usage" message seen with lockdep kernel build.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  arch/x86/kvm/mmu.h     |  2 ++
>  arch/x86/kvm/mmu/mmu.c | 33 +++++++++++++++++++++++++++++++++
>  2 files changed, 35 insertions(+)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index c99b15e97a0a..d55b5166389a 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -178,6 +178,8 @@ static inline bool is_nx_huge_page_enabled(void)
>         return READ_ONCE(nx_huge_pages);
>  }
>
> +bool kvm_mmu_get_tdp_walk(struct kvm_vcpu *vcpu, gpa_t gpa, kvm_pfn_t *pfn, int *level);
> +
>  static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>                                         u32 err, bool prefetch)
>  {
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 569021af349a..c1ac486e096e 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4151,6 +4151,39 @@ kvm_pfn_t kvm_mmu_map_tdp_page(struct kvm_vcpu *vcpu, gpa_t gpa,
>  }
>  EXPORT_SYMBOL_GPL(kvm_mmu_map_tdp_page);
>
> +bool kvm_mmu_get_tdp_walk(struct kvm_vcpu *vcpu, gpa_t gpa, kvm_pfn_t *pfn, int *level)
> +{
> +       u64 sptes[PT64_ROOT_MAX_LEVEL + 1];
> +       int leaf, root;
> +
> +       walk_shadow_page_lockless_begin(vcpu);
> +
> +       if (is_tdp_mmu(vcpu->arch.mmu))
> +               leaf = kvm_tdp_mmu_get_walk(vcpu, gpa, sptes, &root);
> +       else
> +               leaf = get_walk(vcpu, gpa, sptes, &root);
> +
> +       walk_shadow_page_lockless_end(vcpu);
> +
> +       if (unlikely(leaf < 0))
> +               return false;
> +
> +       /* Check if the leaf SPTE is present */
> +       if (!is_shadow_present_pte(sptes[leaf]))
> +               return false;
> +
> +       *pfn = spte_to_pfn(sptes[leaf]);
> +       if (leaf > PG_LEVEL_4K) {
> +               u64 page_mask = KVM_PAGES_PER_HPAGE(leaf) - KVM_PAGES_PER_HPAGE(leaf - 1);
> +               *pfn |= (gpa_to_gfn(gpa) & page_mask);

Similar to the discussion in the other patch, I believe you should
apply the same fix here as well.
It should be
u64 page_mask = KVM_PAGES_PER_HPAGE(leaf) - 1;

> +       }
> +
> +       *level = leaf;
> +
> +       return true;
> +}
> +EXPORT_SYMBOL_GPL(kvm_mmu_get_tdp_walk);
> +
>  static void nonpaging_init_context(struct kvm_mmu *context)
>  {
>         context->page_fault = nonpaging_page_fault;
> --
> 2.25.1
>