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[34.127.75.226]) by smtp.gmail.com with ESMTPSA id p10-20020a637f4a000000b00373a2760775sm2813386pgn.2.2022.03.03.16.07.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Mar 2022 16:07:45 -0800 (PST) Date: Fri, 4 Mar 2022 00:07:41 +0000 From: Mingwei Zhang To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , David Hildenbrand , David Matlack , Ben Gardon Subject: Re: [PATCH v4 16/30] KVM: x86/mmu: Add dedicated helper to zap TDP MMU root shadow page Message-ID: References: <20220303193842.370645-1-pbonzini@redhat.com> <20220303193842.370645-17-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220303193842.370645-17-pbonzini@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 03, 2022, Paolo Bonzini wrote: > From: Sean Christopherson > > Add a dedicated helper for zapping a TDP MMU root, and use it in the three > flows that do "zap_all" and intentionally do not do a TLB flush if SPTEs > are zapped (zapping an entire root is safe if and only if it cannot be in > use by any vCPU). Because a TLB flush is never required, unconditionally > pass "false" to tdp_mmu_iter_cond_resched() when potentially yielding. > > Opportunistically document why KVM must not yield when zapping roots that > are being zapped by kvm_tdp_mmu_put_root(), i.e. roots whose refcount has > reached zero, and further harden the flow to detect improper KVM behavior > with respect to roots that are supposed to be unreachable. > > In addition to hardening zapping of roots, isolating zapping of roots > will allow future simplification of zap_gfn_range() by having it zap only > leaf SPTEs, and by removing its tricky "zap all" heuristic. By having > all paths that truly need to free _all_ SPs flow through the dedicated > root zapper, the generic zapper can be freed of those concerns. > > Signed-off-by: Sean Christopherson > Reviewed-by: Ben Gardon > Message-Id: <20220226001546.360188-16-seanjc@google.com> > Signed-off-by: Paolo Bonzini Reviewed-by: Mingwei Zhang > --- > arch/x86/kvm/mmu/tdp_mmu.c | 98 +++++++++++++++++++++++++++++++------- > 1 file changed, 82 insertions(+), 16 deletions(-) > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c > index f59f3ff5cb75..970376297b30 100644 > --- a/arch/x86/kvm/mmu/tdp_mmu.c > +++ b/arch/x86/kvm/mmu/tdp_mmu.c > @@ -56,10 +56,6 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm) > rcu_barrier(); > } > > -static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root, > - gfn_t start, gfn_t end, bool can_yield, bool flush, > - bool shared); > - > static void tdp_mmu_free_sp(struct kvm_mmu_page *sp) > { > free_page((unsigned long)sp->spt); > @@ -82,6 +78,9 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head) > tdp_mmu_free_sp(sp); > } > > +static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root, > + bool shared); > + > void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root, > bool shared) > { > @@ -104,7 +103,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root, > * intermediate paging structures, that may be zapped, as such entries > * are associated with the ASID on both VMX and SVM. > */ > - (void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared); > + tdp_mmu_zap_root(kvm, root, shared); > > call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback); > } > @@ -737,6 +736,76 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm, > return iter->yielded; > } > > +static inline gfn_t tdp_mmu_max_gfn_host(void) > +{ > + /* > + * Bound TDP MMU walks at host.MAXPHYADDR, guest accesses beyond that > + * will hit a #PF(RSVD) and never hit an EPT Violation/Misconfig / #NPF, > + * and so KVM will never install a SPTE for such addresses. > + */ > + return 1ULL << (shadow_phys_bits - PAGE_SHIFT); > +} > + > +static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root, > + bool shared) > +{ > + bool root_is_unreachable = !refcount_read(&root->tdp_mmu_root_count); > + struct tdp_iter iter; > + > + gfn_t end = tdp_mmu_max_gfn_host(); > + gfn_t start = 0; > + > + kvm_lockdep_assert_mmu_lock_held(kvm, shared); > + > + rcu_read_lock(); > + > + /* > + * No need to try to step down in the iterator when zapping an entire > + * root, zapping an upper-level SPTE will recurse on its children. > + */ > + for_each_tdp_pte_min_level(iter, root, root->role.level, start, end) { > +retry: > + /* > + * Yielding isn't allowed when zapping an unreachable root as > + * the root won't be processed by mmu_notifier callbacks. When > + * handling an unmap/release mmu_notifier command, KVM must > + * drop all references to relevant pages prior to completing > + * the callback. Dropping mmu_lock can result in zapping SPTEs > + * for an unreachable root after a relevant callback completes, > + * which leads to use-after-free as zapping a SPTE triggers > + * "writeback" of dirty/accessed bits to the SPTE's associated > + * struct page. > + */ > + if (!root_is_unreachable && > + tdp_mmu_iter_cond_resched(kvm, &iter, false, shared)) > + continue; > + > + if (!is_shadow_present_pte(iter.old_spte)) > + continue; > + > + if (!shared) { > + tdp_mmu_set_spte(kvm, &iter, 0); > + } else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0)) { > + /* > + * cmpxchg() shouldn't fail if the root is unreachable. > + * Retry so as not to leak the page and its children. > + */ > + WARN_ONCE(root_is_unreachable, > + "Contended TDP MMU SPTE in unreachable root."); > + goto retry; > + } > + > + /* > + * WARN if the root is invalid and is unreachable, all SPTEs > + * should've been zapped by kvm_tdp_mmu_zap_invalidated_roots(), > + * and inserting new SPTEs under an invalid root is a KVM bug. > + */ > + WARN_ON_ONCE(root_is_unreachable && root->role.invalid); > + } > + > + rcu_read_unlock(); > +} > + > bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp) > { > u64 old_spte; > @@ -785,8 +854,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root, > gfn_t start, gfn_t end, bool can_yield, bool flush, > bool shared) > { > - gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT); > - bool zap_all = (start == 0 && end >= max_gfn_host); > + bool zap_all = (start == 0 && end >= tdp_mmu_max_gfn_host()); > struct tdp_iter iter; > > /* > @@ -795,12 +863,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root, > */ > int min_level = zap_all ? root->role.level : PG_LEVEL_4K; > > - /* > - * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will > - * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF, > - * and so KVM will never install a SPTE for such addresses. > - */ > - end = min(end, max_gfn_host); > + end = min(end, tdp_mmu_max_gfn_host()); > > kvm_lockdep_assert_mmu_lock_held(kvm, shared); > > @@ -860,6 +923,7 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start, > > void kvm_tdp_mmu_zap_all(struct kvm *kvm) > { > + struct kvm_mmu_page *root; > int i; > > /* > @@ -867,8 +931,10 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm) > * is being destroyed or the userspace VMM has exited. In both cases, > * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request. > */ > - for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) > - (void)kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, false); > + for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) { > + for_each_tdp_mmu_root_yield_safe(kvm, root, i) > + tdp_mmu_zap_root(kvm, root, false); > + } > } > > static struct kvm_mmu_page *next_invalidated_root(struct kvm *kvm, > @@ -925,7 +991,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm) > * will still flush on yield, but that's a minor performance > * blip and not a functional issue. > */ > - (void)zap_gfn_range(kvm, root, 0, -1ull, true, false, true); > + tdp_mmu_zap_root(kvm, root, true); > > /* > * Put the reference acquired in > -- > 2.31.1 > >