[PATCH v2] KVM: x86: use again the flush argument of __link_shadow_page()

[PATCH v2] KVM: x86: use again the flush argument of __link_shadow_page()

[Paolo Bonzini]

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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 892246204435..f0144ae8d891 100644
--- 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));
-- 
2.54.0

Re: [PATCH v2] KVM: x86: use again the flush argument of __link_shadow_page()

[Sean Christopherson]

On Tue, May 12, 2026, Paolo Bonzini wrote:
> 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.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>