[PATCH v12 41/84] KVM: x86/mmu: Mark pages/folios dirty at the origin of make_spte()

[Sean Christopherson]

Move the marking of folios dirty from make_spte() out to its callers,
which have access to the _struct page_, not just the underlying pfn.
Once all architectures follow suit, this will allow removing KVM's ugly
hack where KVM elevates the refcount of VM_MIXEDMAP pfns that happen to
be struct page memory.

Signed-off-by: Sean Christopherson <seanjc at google.com>
---
 arch/x86/kvm/mmu/mmu.c         | 29 +++++++++++++++++++++++++++--
 arch/x86/kvm/mmu/paging_tmpl.h |  5 +++++
 arch/x86/kvm/mmu/spte.c        | 11 -----------
 3 files changed, 32 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1cdd67707461..7e7b855ce1e1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2918,7 +2918,16 @@ static bool kvm_mmu_prefetch_sptes(struct kvm_vcpu *vcpu, gfn_t gfn, u64 *sptep,
 	for (i = 0; i < nr_pages; i++, gfn++, sptep++) {
 		mmu_set_spte(vcpu, slot, sptep, access, gfn,
 			     page_to_pfn(pages[i]), NULL);
-		kvm_release_page_clean(pages[i]);
+
+		/*
+		 * KVM always prefetches writable pages from the primary MMU,
+		 * and KVM can make its SPTE writable in the fast page, without
+		 * notifying the primary MMU.  Mark pages/folios dirty now to
+		 * ensure file data is written back if it ends up being written
+		 * by the guest.  Because KVM's prefetching GUPs writable PTEs,
+		 * the probability of unnecessary writeback is extremely low.
+		 */
+		kvm_release_page_dirty(pages[i]);
 	}
 
 	return true;
@@ -4314,7 +4323,23 @@ static u8 kvm_max_private_mapping_level(struct kvm *kvm, kvm_pfn_t pfn,
 static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
 				      struct kvm_page_fault *fault, int r)
 {
-	kvm_release_pfn_clean(fault->pfn);
+	lockdep_assert_once(lockdep_is_held(&vcpu->kvm->mmu_lock) ||
+			    r == RET_PF_RETRY);
+
+	/*
+	 * If the page that KVM got from the *primary MMU* is writable, and KVM
+	 * installed or reused a SPTE, mark the page/folio dirty.  Note, this
+	 * may mark a folio dirty even if KVM created a read-only SPTE, e.g. if
+	 * the GFN is write-protected.  Folios can't be safely marked dirty
+	 * outside of mmu_lock as doing so could race with writeback on the
+	 * folio.  As a result, KVM can't mark folios dirty in the fast page
+	 * fault handler, and so KVM must (somewhat) speculatively mark the
+	 * folio dirty if KVM could locklessly make the SPTE writable.
+	 */
+	if (!fault->map_writable || r == RET_PF_RETRY)
+		kvm_release_pfn_clean(fault->pfn);
+	else
+		kvm_release_pfn_dirty(fault->pfn);
 }
 
 static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index b6897916c76b..2e2d87a925ac 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -953,6 +953,11 @@ static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
 		  spte_to_pfn(spte), spte, true, false,
 		  host_writable, &spte);
 
+	/*
+	 * There is no need to mark the pfn dirty, as the new protections must
+	 * be a subset of the old protections, i.e. synchronizing a SPTE cannot
+	 * change the SPTE from read-only to writable.
+	 */
 	return mmu_spte_update(sptep, spte);
 }
 
diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 9b8795bd2f04..2c5650390d3b 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -277,17 +277,6 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 		mark_page_dirty_in_slot(vcpu->kvm, slot, gfn);
 	}
 
-	/*
-	 * If the page that KVM got from the primary MMU is writable, i.e. if
-	 * it's host-writable, mark the page/folio dirty.  As alluded to above,
-	 * folios can't be safely marked dirty in the fast page fault handler,
-	 * and so KVM must (somewhat) speculatively mark the folio dirty even
-	 * though it isn't guaranteed to be written as KVM won't mark the folio
-	 * dirty if/when the SPTE is made writable.
-	 */
-	if (host_writable)
-		kvm_set_pfn_dirty(pfn);
-
 	*new_spte = spte;
 	return wrprot;
 }
-- 
2.46.0.rc1.232.g9752f9e123-goog