KVM: x86: use again the flush argument of __link_shadow_page()

Except in the case of parentless nested-TDP pages, mmu_page_zap_pte()
clears the SPTE but leaves the invalid_list empty.  In this case, using
kvm_flush_remote_tlbs() as kvm_mmu_remote_flush_or_zap() does is overkill.
Avoid flushing the entirety of the remote TLBs unless the invalid_list
was populated: instead, use a more efficient gfn-targeting flush (if
available) and skip it altogether if the caller guarantees that a TLB
flush is not necessary.

Based-on: <20260503201029.106481-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20260503210917.121840-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 892246204435c585929f523143832e4c5425c68f..f0144ae8d891d3462870ae17f1c182f7ccd08bf9 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2526,6 +2526,23 @@ static void shadow_walk_next(struct kvm_shadow_walk_iterator *iterator)
        __shadow_walk_next(iterator, *iterator->sptep);
 }
 
+/*
+ * Note: while normally KVM uses a "bool flush" return value to let
+ * the caller batch flushes, __link_shadow_page() flushes immediately
+ * before populating the parent PTE with the new shadow page.  The
+ * typical callers, direct_map() and FNAME(fetch)(), are not going
+ * to zap more than one huge SPTE anyway.
+ *
+ * The only exception, where @flush can be false, is when a huge SPTE
+ * is replaced with a shadow page SPTE with a fully populated page table,
+ * which can happen from shadow_mmu_split_huge_page().  In this case,
+ * no memory is unmapped across the change to the page tables and no
+ * immediate flush is needed for correctness.
+ *
+ * Even in that case, calls to kvm_mmu_commit_zap_page() are not
+ * batched.  Doing so would require adding an invalid_list argument
+ * all the way down to __walk_slot_rmaps().
+ */
 static void __link_shadow_page(struct kvm *kvm,
                               struct kvm_mmu_memory_cache *cache, u64 *sptep,
                               struct kvm_mmu_page *sp, bool flush)
@@ -2541,8 +2558,10 @@ static void __link_shadow_page(struct kvm *kvm,
                parent_sp = sptep_to_sp(sptep);
                WARN_ON_ONCE(parent_sp->role.level == PG_LEVEL_4K);
 
-               mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list);
-               kvm_mmu_remote_flush_or_zap(kvm, &invalid_list, true);
+               if (mmu_page_zap_pte(kvm, parent_sp, sptep, &invalid_list))
+                       kvm_mmu_commit_zap_page(kvm, &invalid_list);
+               else if (flush)
+                       kvm_flush_remote_tlbs_sptep(kvm, sptep);
        }
 
        spte = make_nonleaf_spte(sp->spt, sp_ad_disabled(sp));