Re: [PATCH V2 5/7] KVM: X86/MMU: Remove the check of the return value of to_shadow_page()

David Matlack
2022-05-17T16:48:05Z

On Tue, May 03, 2022 at 11:07:33PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
>
> Remove the check of the return value of to_shadow_page() in
> mmu_free_root_page(), kvm_mmu_free_guest_mode_roots(), is_unsync_root()
> and is_tdp_mmu() because it can not return NULL.
>
> Remove the check of the return value of to_shadow_page() in
> is_page_fault_stale() and is_obsolete_root() because it can not return
> NULL and the obsoleting for special shadow page is already handled by
> a different way.
>
> When the obsoleting process is done, all the obsoleted shadow pages are
> already unlinked from the special pages by the help of the parent rmap
> of the children and the special pages become theoretically valid again.
> The special shadow page can be freed if is_obsolete_sp() return true,
> or be reused if is_obsolete_sp() return false.
>
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>

Reviewed-by: David Matlack <dmatlack@google.com>

> ---
> arch/x86/kvm/mmu/mmu.c | 44 +++-----------------------------------
> arch/x86/kvm/mmu/tdp_mmu.h | 7 +-----
> 2 files changed, 4 insertions(+), 47 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f626d7e8ebb..bcb3e2730277 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3318,8 +3318,6 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> return;
>
> sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> - if (WARN_ON(!sp))
> - return;
>
> if (is_tdp_mmu_page(sp))
> kvm_tdp_mmu_put_root(kvm, sp, false);
> @@ -3422,8 +3420,7 @@ void kvm_mmu_free_guest_mode_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> if (!VALID_PAGE(root_hpa))
> continue;
>
> - if (!to_shadow_page(root_hpa) ||
> - to_shadow_page(root_hpa)->role.guest_mode)
> + if (to_shadow_page(root_hpa)->role.guest_mode)
> roots_to_free |= KVM_MMU_ROOT_PREVIOUS(i);
> }
>
> @@ -3673,13 +3670,6 @@ static bool is_unsync_root(hpa_t root)
> smp_rmb();
> sp = to_shadow_page(root);
>
> - /*
> - * PAE roots (somewhat arbitrarily) aren't backed by shadow pages, the
> - * PDPTEs for a given PAE root need to be synchronized individually.
> - */
> - if (WARN_ON_ONCE(!sp))
> - return false;
> -
> if (sp->unsync || sp->unsync_children)
> return true;
>
> @@ -3975,21 +3965,7 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
> struct kvm_page_fault *fault, int mmu_seq)
> {
> - struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root.hpa);
> -
> - /* Special roots, e.g. pae_root, are not backed by shadow pages. */
> - if (sp && is_obsolete_sp(vcpu->kvm, sp))
> - return true;
> -
> - /*
> - * Roots without an associated shadow page are considered invalid if
> - * there is a pending request to free obsolete roots. The request is
> - * only a hint that the current root _may_ be obsolete and needs to be
> - * reloaded, e.g. if the guest frees a PGD that KVM is tracking as a
> - * previous root, then __kvm_mmu_prepare_zap_page() signals all vCPUs
> - * to reload even if no vCPU is actively using the root.
> - */
> - if (!sp && kvm_test_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu))
> + if (is_obsolete_sp(vcpu->kvm, to_shadow_page(vcpu->arch.mmu->root.hpa)))
> return true;
>
> return fault->slot &&
> @@ -5094,24 +5070,10 @@ void kvm_mmu_unload(struct kvm_vcpu *vcpu)
>
> static bool is_obsolete_root(struct kvm *kvm, hpa_t root_hpa)
> {
> - struct kvm_mmu_page *sp;
> -
> if (!VALID_PAGE(root_hpa))
> return false;
>
> - /*
> - * When freeing obsolete roots, treat roots as obsolete if they don't
> - * have an associated shadow page. This does mean KVM will get false
> - * positives and free roots that don't strictly need to be freed, but
> - * such false positives are relatively rare:
> - *
> - * (a) only PAE paging and nested NPT has roots without shadow pages
> - * (b) remote reloads due to a memslot update obsoletes _all_ roots
> - * (c) KVM doesn't track previous roots for PAE paging, and the guest
> - * is unlikely to zap an in-use PGD.
> - */
> - sp = to_shadow_page(root_hpa);
> - return !sp || is_obsolete_sp(kvm, sp);
> + return is_obsolete_sp(kvm, to_shadow_page(root_hpa));
> }
>
> static void __kvm_mmu_free_obsolete_roots(struct kvm *kvm, struct kvm_mmu *mmu)
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index c163f7cc23ca..5779a2a7161e 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -78,13 +78,8 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
> if (WARN_ON(!VALID_PAGE(hpa)))
> return false;
>
> - /*
> - * A NULL shadow page is legal when shadowing a non-paging guest with
> - * PAE paging, as the MMU will be direct with root_hpa pointing at the
> - * pae_root page, not a shadow page.
> - */
> sp = to_shadow_page(hpa);
> - return sp && is_tdp_mmu_page(sp) && sp->root_count;
> + return is_tdp_mmu_page(sp) && sp->root_count;
> }
> #else
> static inline int kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return 0; }
> --
> 2.19.1.6.gb485710b
>