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Date: Thu, 11 Nov 2021 18:06:57 +0000
From: David Matlack <dmatlack@...gle.com>
To: Ben Gardon <bgardon@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Xu <peterx@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Peter Shier <pshier@...gle.com>,
Mingwei Zhang <mizhang@...gle.com>,
Yulei Zhang <yulei.kernel@...il.com>,
Wanpeng Li <kernellwp@...il.com>,
Xiao Guangrong <xiaoguangrong.eric@...il.com>,
Kai Huang <kai.huang@...el.com>,
Keqian Zhu <zhukeqian1@...wei.com>,
David Hildenbrand <david@...hat.com>
Subject: Re: [RFC 02/19] KVM: x86/mmu: Batch TLB flushes for a single zap
On Wed, Nov 10, 2021 at 02:29:53PM -0800, Ben Gardon wrote:
> When recursively handling a removed TDP page table, the TDP MMU will
> flush the TLBs and queue an RCU callback to free the PT. If the original
> change zapped a non-leaf SPTE at PG_LEVEL_1G or above, that change will
> result in many unnecessary TLB flushes when one would suffice. Queue all
> the PTs which need to be freed on a list and wait to queue RCU callbacks
> to free them until after all the recursive callbacks are done.
>
>
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 88 ++++++++++++++++++++++++++++++--------
> 1 file changed, 70 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 866c2b191e1e..5b31d046df78 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -220,7 +220,8 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
>
> static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> u64 old_spte, u64 new_spte, int level,
> - bool shared);
> + bool shared,
> + struct list_head *disconnected_sps);
>
> static void handle_changed_spte_acc_track(u64 old_spte, u64 new_spte, int level)
> {
> @@ -302,6 +303,11 @@ static void tdp_mmu_unlink_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> * @shared: This operation may not be running under the exclusive use
> * of the MMU lock and the operation must synchronize with other
> * threads that might be modifying SPTEs.
> + * @disconnected_sps: If null, the TLBs will be flushed and the disconnected
> + * TDP MMU page will be queued to be freed after an RCU
> + * callback. If non-null the page will be added to the list
> + * and flushing the TLBs and queueing an RCU callback to
> + * free the page will be the caller's responsibility.
> *
> * Given a page table that has been removed from the TDP paging structure,
> * iterates through the page table to clear SPTEs and free child page tables.
> @@ -312,7 +318,8 @@ static void tdp_mmu_unlink_page(struct kvm *kvm, struct kvm_mmu_page *sp,
> * early rcu_dereferences in the function.
> */
> static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> - bool shared)
> + bool shared,
> {
> struct kvm_mmu_page *sp = sptep_to_sp(rcu_dereference(pt));
> int level = sp->role.level;
> @@ -371,13 +378,16 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> }
> handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
> old_child_spte, REMOVED_SPTE, level,
> - shared);
> + shared, disconnected_sps);
> }
>
> - kvm_flush_remote_tlbs_with_address(kvm, base_gfn,
> - KVM_PAGES_PER_HPAGE(level + 1));
> -
> - call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> + if (disconnected_sps) {
> + list_add_tail(&sp->link, disconnected_sps);
> + } else {
> + kvm_flush_remote_tlbs_with_address(kvm, base_gfn,
> + KVM_PAGES_PER_HPAGE(level + 1));
> + call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> + }
> }
>
> /**
> @@ -391,13 +401,21 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> * @shared: This operation may not be running under the exclusive use of
> * the MMU lock and the operation must synchronize with other
> * threads that might be modifying SPTEs.
> + * @disconnected_sps: Only used if a page of page table memory has been
> + * removed from the paging structure by this change.
> + * If null, the TLBs will be flushed and the disconnected
> + * TDP MMU page will be queued to be freed after an RCU
> + * callback. If non-null the page will be added to the list
> + * and flushing the TLBs and queueing an RCU callback to
> + * free the page will be the caller's responsibility.
> *
> * Handle bookkeeping that might result from the modification of a SPTE.
> * This function must be called for all TDP SPTE modifications.
> */
> static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> u64 old_spte, u64 new_spte, int level,
> - bool shared)
> + bool shared,
> + struct list_head *disconnected_sps)
> {
> bool was_present = is_shadow_present_pte(old_spte);
> bool is_present = is_shadow_present_pte(new_spte);
> @@ -475,22 +493,39 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> */
> if (was_present && !was_leaf && (pfn_changed || !is_present))
> handle_removed_tdp_mmu_page(kvm,
> - spte_to_child_pt(old_spte, level), shared);
> + spte_to_child_pt(old_spte, level), shared,
> + disconnected_sps);
> }
>
> static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> u64 old_spte, u64 new_spte, int level,
> - bool shared)
> + bool shared, struct list_head *disconnected_sps)
> {
> __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level,
> - shared);
> + shared, disconnected_sps);
> handle_changed_spte_acc_track(old_spte, new_spte, level);
> handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
> new_spte, level);
> }
>
> /*
> - * tdp_mmu_set_spte_atomic - Set a TDP MMU SPTE atomically
> + * The TLBs must be flushed between the pages linked from disconnected_sps
> + * being removed from the paging structure and this function being called.
> + */
> +static void handle_disconnected_sps(struct kvm *kvm,
> + struct list_head *disconnected_sps)
handle_disconnected_sps() does a very specific task so I think we could
go with a more specific function name to make the code more readable.
How about free_sps_rcu() or call_rcu_free_sps()?
> +{
> + struct kvm_mmu_page *sp;
> + struct kvm_mmu_page *next;
> +
> + list_for_each_entry_safe(sp, next, disconnected_sps, link) {
> + list_del(&sp->link);
> + call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> + }
> +}
> +
> +/*
> + * __tdp_mmu_set_spte_atomic - Set a TDP MMU SPTE atomically
> * and handle the associated bookkeeping. Do not mark the page dirty
> * in KVM's dirty bitmaps.
> *
> @@ -500,9 +535,10 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> * Returns: true if the SPTE was set, false if it was not. If false is returned,
> * this function will have no side-effects.
> */
> -static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> - struct tdp_iter *iter,
> - u64 new_spte)
> +static inline bool __tdp_mmu_set_spte_atomic(struct kvm *kvm,
> + struct tdp_iter *iter,
> + u64 new_spte,
> + struct list_head *disconnected_sps)
> {
> lockdep_assert_held_read(&kvm->mmu_lock);
>
> @@ -522,22 +558,32 @@ static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> return false;
>
> __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
> - new_spte, iter->level, true);
> + new_spte, iter->level, true, disconnected_sps);
> handle_changed_spte_acc_track(iter->old_spte, new_spte, iter->level);
>
> return true;
> }
>
> +static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> + struct tdp_iter *iter,
> + u64 new_spte)
> +{
> + return __tdp_mmu_set_spte_atomic(kvm, iter, new_spte, NULL);
Why not leverage disconnected_sps here as well? Then you can remove the
NULL case (and comments) from handle_removed_tdp_mmu_page.
> +}
> +
> static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
> struct tdp_iter *iter)
> {
> + LIST_HEAD(disconnected_sps);
> +
> /*
> * Freeze the SPTE by setting it to a special,
> * non-present value. This will stop other threads from
> * immediately installing a present entry in its place
> * before the TLBs are flushed.
> */
> - if (!tdp_mmu_set_spte_atomic(kvm, iter, REMOVED_SPTE))
> + if (!__tdp_mmu_set_spte_atomic(kvm, iter, REMOVED_SPTE,
> + &disconnected_sps))
> return false;
>
> kvm_flush_remote_tlbs_with_address(kvm, iter->gfn,
> @@ -553,6 +599,8 @@ static inline bool tdp_mmu_zap_spte_atomic(struct kvm *kvm,
> */
> WRITE_ONCE(*rcu_dereference(iter->sptep), 0);
>
> + handle_disconnected_sps(kvm, &disconnected_sps);
> +
> return true;
> }
>
> @@ -577,6 +625,8 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> u64 new_spte, bool record_acc_track,
> bool record_dirty_log)
> {
> + LIST_HEAD(disconnected_sps);
> +
> lockdep_assert_held_write(&kvm->mmu_lock);
>
> /*
> @@ -591,7 +641,7 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> WRITE_ONCE(*rcu_dereference(iter->sptep), new_spte);
>
> __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
> - new_spte, iter->level, false);
> + new_spte, iter->level, false, &disconnected_sps);
> if (record_acc_track)
> handle_changed_spte_acc_track(iter->old_spte, new_spte,
> iter->level);
> @@ -599,6 +649,8 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> handle_changed_spte_dirty_log(kvm, iter->as_id, iter->gfn,
> iter->old_spte, new_spte,
> iter->level);
> +
> + handle_disconnected_sps(kvm, &disconnected_sps);
Where is the TLB flush for these disconnected_sps?
> }
>
> static inline void tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> --
> 2.34.0.rc0.344.g81b53c2807-goog
>
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