KVM: x86/mmu: Yield in TDU MMU iter even if no SPTES changed

Given certain conditions, some TDP MMU functions may not yield
reliably / frequently enough. For example, if a paging structure was
very large but had few, if any writable entries, wrprot_gfn_range
could traverse many entries before finding a writable entry and yielding
because the check for yielding only happens after an SPTE is modified.

Fix this issue by moving the yield to the beginning of the loop.

Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
Reviewed-by: Peter Feiner <pfeiner@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-15-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7cfc063..c8a1149 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -501,6 +501,12 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
        bool flush_needed = false;
 
        tdp_root_for_each_pte(iter, root, start, end) {
+               if (can_yield &&
+                   tdp_mmu_iter_cond_resched(kvm, &iter, flush_needed)) {
+                       flush_needed = false;
+                       continue;
+               }
+
                if (!is_shadow_present_pte(iter.old_spte))
                        continue;
 
@@ -515,9 +521,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
                        continue;
 
                tdp_mmu_set_spte(kvm, &iter, 0);
-
-               flush_needed = !(can_yield &&
-                                tdp_mmu_iter_cond_resched(kvm, &iter, true));
+               flush_needed = true;
        }
        return flush_needed;
 }
@@ -880,6 +884,9 @@ 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))
+                       continue;
+
                if (!is_shadow_present_pte(iter.old_spte) ||
                    !is_last_spte(iter.old_spte, iter.level))
                        continue;
@@ -888,8 +895,6 @@ static bool wrprot_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 
                tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
                spte_set = true;
-
-               tdp_mmu_iter_cond_resched(kvm, &iter, false);
        }
        return spte_set;
 }
@@ -933,6 +938,9 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
        bool spte_set = false;
 
        tdp_root_for_each_leaf_pte(iter, root, start, end) {
+               if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
+                       continue;
+
                if (spte_ad_need_write_protect(iter.old_spte)) {
                        if (is_writable_pte(iter.old_spte))
                                new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
@@ -947,8 +955,6 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 
                tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
                spte_set = true;
-
-               tdp_mmu_iter_cond_resched(kvm, &iter, false);
        }
        return spte_set;
 }
@@ -1056,6 +1062,9 @@ static bool set_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
        bool spte_set = false;
 
        tdp_root_for_each_pte(iter, root, start, end) {
+               if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
+                       continue;
+
                if (!is_shadow_present_pte(iter.old_spte))
                        continue;
 
@@ -1063,8 +1072,6 @@ static bool set_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 
                tdp_mmu_set_spte(kvm, &iter, new_spte);
                spte_set = true;
-
-               tdp_mmu_iter_cond_resched(kvm, &iter, false);
        }
 
        return spte_set;
@@ -1105,6 +1112,11 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
        bool spte_set = false;
 
        tdp_root_for_each_pte(iter, root, start, end) {
+               if (tdp_mmu_iter_cond_resched(kvm, &iter, spte_set)) {
+                       spte_set = false;
+                       continue;
+               }
+
                if (!is_shadow_present_pte(iter.old_spte) ||
                    !is_last_spte(iter.old_spte, iter.level))
                        continue;
@@ -1116,7 +1128,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
 
                tdp_mmu_set_spte(kvm, &iter, 0);
 
-               spte_set = !tdp_mmu_iter_cond_resched(kvm, &iter, true);
+               spte_set = true;
        }
 
        if (spte_set)