[v13,41/85] KVM: x86/mmu: Mark pages/folios dirty at the origin of make_spte()

Commit Message

Sean Christopherson Oct. 10, 2024, 6:23 p.m. UTC

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.

Tested-by: Alex Bennée <[e-mail]>
Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/mmu/mmu.c         | 30 ++++++++++++++++++++++++++++--
 arch/x86/kvm/mmu/paging_tmpl.h |  5 +++++
 arch/x86/kvm/mmu/spte.c        | 11 -----------
 3 files changed, 33 insertions(+), 13 deletions(-)

Patch

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 31a6ae41a6f4..f730870887dd 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2964,7 +2964,17 @@  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 handler,
+		 * 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;
@@ -4360,7 +4370,23 @@  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 35d0c3f1a789..f4711674c47b 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -954,6 +954,11 @@  static int FNAME(sync_spte)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, int
 		  spte_to_pfn(spte), spte, true, true,
 		  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 8e8d6ee79c8b..f1a50a78badb 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;
 }