Re: [PATCH 09/13] KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock

[Ben Gardon]

On Thu, Apr 1, 2021 at 2:58 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 31/03/21 23:08, Ben Gardon wrote:
> > To reduce lock contention and interference with page fault handlers,
> > allow the TDP MMU function to zap a GFN range to operate under the MMU
> > read lock.
> >
> > Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mmu/mmu.c     |  15 ++++--
> >   arch/x86/kvm/mmu/tdp_mmu.c | 102 ++++++++++++++++++++++++++-----------
> >   arch/x86/kvm/mmu/tdp_mmu.h |   6 ++-
> >   3 files changed, 87 insertions(+), 36 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 667d64daa82c..dcbfc784cf2f 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3155,7 +3155,7 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> >       sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
> >
> >       if (is_tdp_mmu_page(sp))
> > -             kvm_tdp_mmu_put_root(kvm, sp);
> > +             kvm_tdp_mmu_put_root(kvm, sp, false);
> >       else if (!--sp->root_count && sp->role.invalid)
> >               kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
> >
> > @@ -5514,13 +5514,17 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> >               }
> >       }
> >
> > +     write_unlock(&kvm->mmu_lock);
> > +
> >       if (is_tdp_mmu_enabled(kvm)) {
> > -             flush = kvm_tdp_mmu_zap_gfn_range(kvm, gfn_start, gfn_end);
> > +             read_lock(&kvm->mmu_lock);
> > +             flush = kvm_tdp_mmu_zap_gfn_range(kvm, gfn_start, gfn_end,
> > +                                               true);
> >               if (flush)
> >                       kvm_flush_remote_tlbs(kvm);
> > -     }
> >
> > -     write_unlock(&kvm->mmu_lock);
> > +             read_unlock(&kvm->mmu_lock);
> > +     }
> >   }
>
> This will conflict with Sean's MMU notifier series patches:
>
> KVM: x86/mmu: Pass address space ID to __kvm_tdp_mmu_zap_gfn_range()
>
> What I can do for now is change the mmu.c part of that patch to
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e6e02360ef67..9882bbd9b742 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5510,15 +5510,15 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
>                 }
>         }
>
> -       if (flush)
> -               kvm_flush_remote_tlbs_with_address(kvm, gfn_start, gfn_end);
> -
>         if (is_tdp_mmu_enabled(kvm)) {
> -               flush = kvm_tdp_mmu_zap_gfn_range(kvm, gfn_start, gfn_end);
> -               if (flush)
> -                       kvm_flush_remote_tlbs(kvm);
> +               for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> +                       flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start,
> +                                                         gfn_end, flush);
>         }
>
> +       if (flush)
> +               kvm_flush_remote_tlbs_with_address(kvm, gfn_start, gfn_end);
> +
>         write_unlock(&kvm->mmu_lock);
>   }
>
>
> but you will have to add a separate "if (flush)" when moving the write_unlock
> earlier, since there's no downgrade function for rwlocks.  In practice it's
> not a huge deal since unless running nested there will be only one active MMU.
>
> Paolo

Thank you for doing that. I also figured that the extra flushes when
running nested would probably be worth it to get the parallelism
gains. I don't mind working out those conflicts in v2.


>
> >   static bool slot_rmap_write_protect(struct kvm *kvm,
> > @@ -5959,7 +5963,8 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
> >               WARN_ON_ONCE(!sp->lpage_disallowed);
> >               if (is_tdp_mmu_page(sp)) {
> >                       kvm_tdp_mmu_zap_gfn_range(kvm, sp->gfn,
> > -                             sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level));
> > +                             sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level),
> > +                             false);
> >               } else {
> >                       kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
> >                       WARN_ON_ONCE(sp->lpage_disallowed);
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index d255125059c4..0e99e4675dd4 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -27,6 +27,15 @@ void kvm_mmu_init_tdp_mmu(struct kvm *kvm)
> >       INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
> >   }
> >
> > +static __always_inline void kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
> > +                                                          bool shared)
> > +{
> > +     if (shared)
> > +             lockdep_assert_held_read(&kvm->mmu_lock);
> > +     else
> > +             lockdep_assert_held_write(&kvm->mmu_lock);
> > +}
> > +
> >   void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
> >   {
> >       if (!kvm->arch.tdp_mmu_enabled)
> > @@ -42,7 +51,7 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
> >   }
> >
> >   static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> > -                       gfn_t start, gfn_t end, bool can_yield);
> > +                       gfn_t start, gfn_t end, bool can_yield, bool shared);
> >
> >   static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
> >   {
> > @@ -66,11 +75,12 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
> >       tdp_mmu_free_sp(sp);
> >   }
> >
> > -void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
> > +void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > +                       bool shared)
> >   {
> >       gfn_t max_gfn = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> >
> > -     lockdep_assert_held_write(&kvm->mmu_lock);
> > +     kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> >
> >       if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
> >               return;
> > @@ -81,7 +91,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
> >       list_del_rcu(&root->link);
> >       spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> >
> > -     zap_gfn_range(kvm, root, 0, max_gfn, false);
> > +     zap_gfn_range(kvm, root, 0, max_gfn, false, shared);
> >
> >       call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
> >   }
> > @@ -94,11 +104,11 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
> >    * function will return NULL.
> >    */
> >   static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> > -                                           struct kvm_mmu_page *prev_root)
> > +                                           struct kvm_mmu_page *prev_root,
> > +                                           bool shared)
> >   {
> >       struct kvm_mmu_page *next_root;
> >
> > -     lockdep_assert_held_write(&kvm->mmu_lock);
> >
> >       rcu_read_lock();
> >
> > @@ -117,7 +127,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> >       rcu_read_unlock();
> >
> >       if (prev_root)
> > -             kvm_tdp_mmu_put_root(kvm, prev_root);
> > +             kvm_tdp_mmu_put_root(kvm, prev_root, shared);
> >
> >       return next_root;
> >   }
> > @@ -127,11 +137,15 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
> >    * This makes it safe to release the MMU lock and yield within the loop, but
> >    * if exiting the loop early, the caller must drop the reference to the most
> >    * recent root. (Unless keeping a live reference is desirable.)
> > + *
> > + * If shared is set, this function is operating under the MMU lock in read
> > + * mode. In the unlikely event that this thread must free a root, the lock
> > + * will be temporarily dropped and reacquired in write mode.
> >    */
> > -#define for_each_tdp_mmu_root_yield_safe(_kvm, _root)        \
> > -     for (_root = tdp_mmu_next_root(_kvm, NULL);     \
> > -          _root;                                     \
> > -          _root = tdp_mmu_next_root(_kvm, _root))
> > +#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _shared)       \
> > +     for (_root = tdp_mmu_next_root(_kvm, NULL, _shared);    \
> > +          _root;                                             \
> > +          _root = tdp_mmu_next_root(_kvm, _root, _shared))
> >
> >   /* Only safe under the MMU lock in write mode, without yielding. */
> >   #define for_each_tdp_mmu_root(_kvm, _root)                          \
> > @@ -632,7 +646,8 @@ static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
> >    * Return false if a yield was not needed.
> >    */
> >   static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
> > -                                          struct tdp_iter *iter, bool flush)
> > +                                          struct tdp_iter *iter, bool flush,
> > +                                          bool shared)
> >   {
> >       /* Ensure forward progress has been made before yielding. */
> >       if (iter->next_last_level_gfn == iter->yielded_gfn)
> > @@ -644,7 +659,11 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
> >               if (flush)
> >                       kvm_flush_remote_tlbs(kvm);
> >
> > -             cond_resched_rwlock_write(&kvm->mmu_lock);
> > +             if (shared)
> > +                     cond_resched_rwlock_read(&kvm->mmu_lock);
> > +             else
> > +                     cond_resched_rwlock_write(&kvm->mmu_lock);
> > +
> >               rcu_read_lock();
> >
> >               WARN_ON(iter->gfn > iter->next_last_level_gfn);
> > @@ -662,23 +681,33 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
> >    * non-root pages mapping GFNs strictly within that range. Returns true if
> >    * SPTEs have been cleared and a TLB flush is needed before releasing the
> >    * MMU lock.
> > + *
> >    * If can_yield is true, will release the MMU lock and reschedule if the
> >    * scheduler needs the CPU or there is contention on the MMU lock. If this
> >    * function cannot yield, it will not release the MMU lock or reschedule and
> >    * the caller must ensure it does not supply too large a GFN range, or the
> >    * operation can cause a soft lockup.
> > + *
> > + * If shared is true, this thread holds the MMU lock in read mode and must
> > + * account for the possibility that other threads are modifying the paging
> > + * structures concurrently. If shared is false, this thread should hold the
> > + * MMU lock in write mode.
> >    */
> >   static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> > -                       gfn_t start, gfn_t end, bool can_yield)
> > +                       gfn_t start, gfn_t end, bool can_yield, bool shared)
> >   {
> >       struct tdp_iter iter;
> >       bool flush_needed = false;
> >
> > +     kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> > +
> >       rcu_read_lock();
> >
> >       tdp_root_for_each_pte(iter, root, start, end) {
> > +retry:
> >               if (can_yield &&
> > -                 tdp_mmu_iter_cond_resched(kvm, &iter, flush_needed)) {
> > +                 tdp_mmu_iter_cond_resched(kvm, &iter, flush_needed,
> > +                                           shared)) {
> >                       flush_needed = false;
> >                       continue;
> >               }
> > @@ -696,8 +725,17 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> >                   !is_last_spte(iter.old_spte, iter.level))
> >                       continue;
> >
> > -             tdp_mmu_set_spte(kvm, &iter, 0);
> > -             flush_needed = true;
> > +             if (!shared) {
> > +                     tdp_mmu_set_spte(kvm, &iter, 0);
> > +                     flush_needed = true;
> > +             } else if (!tdp_mmu_zap_spte_atomic(kvm, &iter)) {
> > +                     /*
> > +                      * The iter must explicitly re-read the SPTE because
> > +                      * the atomic cmpxchg failed.
> > +                      */
> > +                     iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
> > +                     goto retry;
> > +             }
> >       }
> >
> >       rcu_read_unlock();
> > @@ -709,14 +747,20 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> >    * non-root pages mapping GFNs strictly within that range. Returns true if
> >    * SPTEs have been cleared and a TLB flush is needed before releasing the
> >    * MMU lock.
> > + *
> > + * If shared is true, this thread holds the MMU lock in read mode and must
> > + * account for the possibility that other threads are modifying the paging
> > + * structures concurrently. If shared is false, this thread should hold the
> > + * MMU in write mode.
> >    */
> > -bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, gfn_t start, gfn_t end)
> > +bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, gfn_t start, gfn_t end,
> > +                            bool shared)
> >   {
> >       struct kvm_mmu_page *root;
> >       bool flush = false;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root)
> > -             flush |= zap_gfn_range(kvm, root, start, end, true);
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, shared)
> > +             flush |= zap_gfn_range(kvm, root, start, end, true, shared);
> >
> >       return flush;
> >   }
> > @@ -726,7 +770,7 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
> >       gfn_t max_gfn = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> >       bool flush;
> >
> > -     flush = kvm_tdp_mmu_zap_gfn_range(kvm, 0, max_gfn);
> > +     flush = kvm_tdp_mmu_zap_gfn_range(kvm, 0, max_gfn, false);
> >       if (flush)
> >               kvm_flush_remote_tlbs(kvm);
> >   }
> > @@ -893,7 +937,7 @@ static __always_inline int kvm_tdp_mmu_handle_hva_range(struct kvm *kvm,
> >       int ret = 0;
> >       int as_id;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root) {
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> >               as_id = kvm_mmu_page_as_id(root);
> >               slots = __kvm_memslots(kvm, as_id);
> >               kvm_for_each_memslot(memslot, slots) {
> > @@ -933,7 +977,7 @@ static int zap_gfn_range_hva_wrapper(struct kvm *kvm,
> >                                    struct kvm_mmu_page *root, gfn_t start,
> >                                    gfn_t end, unsigned long unused)
> >   {
> > -     return zap_gfn_range(kvm, root, start, end, false);
> > +     return zap_gfn_range(kvm, root, start, end, false, false);
> >   }
> >
> >   int kvm_tdp_mmu_zap_hva_range(struct kvm *kvm, unsigned long start,
> > @@ -1098,7 +1142,7 @@ static bool wrprot_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> >
> >       for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
> >                                  min_level, start, end) {
> > -             if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
> > +             if (tdp_mmu_iter_cond_resched(kvm, &iter, false, false))
> >                       continue;
> >
> >               if (!is_shadow_present_pte(iter.old_spte) ||
> > @@ -1128,7 +1172,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm, struct kvm_memory_slot *slot,
> >       int root_as_id;
> >       bool spte_set = false;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root) {
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> >               root_as_id = kvm_mmu_page_as_id(root);
> >               if (root_as_id != slot->as_id)
> >                       continue;
> > @@ -1157,7 +1201,7 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> >       rcu_read_lock();
> >
> >       tdp_root_for_each_leaf_pte(iter, root, start, end) {
> > -             if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
> > +             if (tdp_mmu_iter_cond_resched(kvm, &iter, false, false))
> >                       continue;
> >
> >               if (spte_ad_need_write_protect(iter.old_spte)) {
> > @@ -1193,7 +1237,7 @@ bool kvm_tdp_mmu_clear_dirty_slot(struct kvm *kvm, struct kvm_memory_slot *slot)
> >       int root_as_id;
> >       bool spte_set = false;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root) {
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> >               root_as_id = kvm_mmu_page_as_id(root);
> >               if (root_as_id != slot->as_id)
> >                       continue;
> > @@ -1291,7 +1335,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
> >       rcu_read_lock();
> >
> >       tdp_root_for_each_pte(iter, root, start, end) {
> > -             if (tdp_mmu_iter_cond_resched(kvm, &iter, spte_set)) {
> > +             if (tdp_mmu_iter_cond_resched(kvm, &iter, spte_set, false)) {
> >                       spte_set = false;
> >                       continue;
> >               }
> > @@ -1326,7 +1370,7 @@ void kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
> >       struct kvm_mmu_page *root;
> >       int root_as_id;
> >
> > -     for_each_tdp_mmu_root_yield_safe(kvm, root) {
> > +     for_each_tdp_mmu_root_yield_safe(kvm, root, false) {
> >               root_as_id = kvm_mmu_page_as_id(root);
> >               if (root_as_id != slot->as_id)
> >                       continue;
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > index 9961df505067..855e58856815 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > @@ -13,9 +13,11 @@ __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm *kvm,
> >       return refcount_inc_not_zero(&root->tdp_mmu_root_count);
> >   }
> >
> > -void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);
> > +void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
> > +                       bool shared);
> >
> > -bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, gfn_t start, gfn_t end);
> > +bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, gfn_t start, gfn_t end,
> > +                            bool shared);
> >   void kvm_tdp_mmu_zap_all(struct kvm *kvm);
> >
> >   int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> >
>