[PATCH v15 12/21] KVM: x86/mmu: Consult guest_memfd when computing max_mapping_level

[Fuad Tabba <tabba@google.com>]

From: Ackerley Tng <ackerleytng@google.com>

Modify kvm_mmu_max_mapping_level() to consult guest_memfd for memory
regions backed by it when computing the maximum mapping level,
especially during huge page recovery.

Previously, kvm_mmu_max_mapping_level() was designed primarily for
host-backed memory and private pages. With guest_memfd now supporting
non-private memory, it's necessary to factor in guest_memfd's influence
on mapping levels for such memory.

Since guest_memfd can now be used for non-private memory, make
kvm_max_max_mapping_level, when recovering huge pages, take input from
guest_memfd.

Input is taken from guest_memfd as long as a fault to that slot and gfn
would have been served from guest_memfd. For now, take a shortcut if the
slot and gfn points to memory that is private, since recovering huge
pages aren't supported for private memory yet.

Since guest_memfd memory can also be faulted into host page tables,
__kvm_mmu_max_mapping_level() still applies since consulting lpage_info
and host page tables are required.

Move functions kvm_max_level_for_order() and
kvm_gmem_max_mapping_level() so kvm_mmu_max_mapping_level() can use
those functions.

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Co-developed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/x86/kvm/mmu/mmu.c   | 90 ++++++++++++++++++++++++----------------
 include/linux/kvm_host.h |  7 ++++
 virt/kvm/guest_memfd.c   | 17 ++++++++
 3 files changed, 79 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 6bd28fda0fd3..94be15cde6da 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3282,13 +3282,67 @@ static int __kvm_mmu_max_mapping_level(struct kvm *kvm,
 	return min(host_level, max_level);
 }
 
+static u8 kvm_max_level_for_order(int order)
+{
+	BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
+
+	KVM_MMU_WARN_ON(order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G) &&
+			order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M) &&
+			order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_4K));
+
+	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G))
+		return PG_LEVEL_1G;
+
+	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
+		return PG_LEVEL_2M;
+
+	return PG_LEVEL_4K;
+}
+
+static u8 kvm_gmem_max_mapping_level(struct kvm *kvm, int order,
+				     struct kvm_page_fault *fault)
+{
+	u8 req_max_level;
+	u8 max_level;
+
+	max_level = kvm_max_level_for_order(order);
+	if (max_level == PG_LEVEL_4K)
+		return PG_LEVEL_4K;
+
+	req_max_level = kvm_x86_call(max_mapping_level)(kvm, fault);
+	if (req_max_level)
+		max_level = min(max_level, req_max_level);
+
+	return max_level;
+}
+
 int kvm_mmu_max_mapping_level(struct kvm *kvm,
 			      const struct kvm_memory_slot *slot, gfn_t gfn)
 {
 	bool is_private = kvm_slot_has_gmem(slot) &&
 			  kvm_mem_is_private(kvm, gfn);
+	int max_level = PG_LEVEL_NUM;
+
+	/*
+	 * For now, kvm_mmu_max_mapping_level() is only called from
+	 * kvm_mmu_recover_huge_pages(), and that's not yet supported for
+	 * private memory, hence we can take a shortcut and return early.
+	 */
+	if (is_private)
+		return PG_LEVEL_4K;
 
-	return __kvm_mmu_max_mapping_level(kvm, slot, gfn, PG_LEVEL_NUM, is_private);
+	/*
+	 * For non-private pages that would have been faulted from guest_memfd,
+	 * let guest_memfd influence max_mapping_level.
+	 */
+	if (kvm_memslot_is_gmem_only(slot)) {
+		int order = kvm_gmem_mapping_order(slot, gfn);
+
+		max_level = min(max_level,
+				kvm_gmem_max_mapping_level(kvm, order, NULL));
+	}
+
+	return __kvm_mmu_max_mapping_level(kvm, slot, gfn, max_level, is_private);
 }
 
 void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
@@ -4450,40 +4504,6 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
 		vcpu->stat.pf_fixed++;
 }
 
-static inline u8 kvm_max_level_for_order(int order)
-{
-	BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
-
-	KVM_MMU_WARN_ON(order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G) &&
-			order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M) &&
-			order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_4K));
-
-	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G))
-		return PG_LEVEL_1G;
-
-	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
-		return PG_LEVEL_2M;
-
-	return PG_LEVEL_4K;
-}
-
-static u8 kvm_gmem_max_mapping_level(struct kvm *kvm, int order,
-				     struct kvm_page_fault *fault)
-{
-	u8 req_max_level;
-	u8 max_level;
-
-	max_level = kvm_max_level_for_order(order);
-	if (max_level == PG_LEVEL_4K)
-		return PG_LEVEL_4K;
-
-	req_max_level = kvm_x86_call(max_mapping_level)(kvm, fault);
-	if (req_max_level)
-		max_level = min(max_level, req_max_level);
-
-	return max_level;
-}
-
 static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
 				      struct kvm_page_fault *fault, int r)
 {
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d2218ec57ceb..662271314778 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2574,6 +2574,7 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
 		     int *max_order);
+int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot, gfn_t gfn);
 #else
 static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn,
@@ -2583,6 +2584,12 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 	KVM_BUG_ON(1, kvm);
 	return -EIO;
 }
+static inline int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot,
+					 gfn_t gfn)
+{
+	WARN_ONCE(1, "Unexpected call since gmem is disabled.");
+	return 0;
+}
 #endif /* CONFIG_KVM_GMEM */
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 2b00f8796a15..d01bd7a2c2bd 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -713,6 +713,23 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 }
 EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
 
+/**
+ * kvm_gmem_mapping_order() - Get the mapping order for this @gfn in @slot.
+ *
+ * @slot: the memslot that gfn belongs to.
+ * @gfn: the gfn to look up mapping order for.
+ *
+ * This is equal to max_order that would be returned if kvm_gmem_get_pfn() were
+ * called now.
+ *
+ * Return: the mapping order for this @gfn in @slot.
+ */
+int kvm_gmem_mapping_order(const struct kvm_memory_slot *slot, gfn_t gfn)
+{
+	return 0;
+}
+EXPORT_SYMBOL_GPL(kvm_gmem_mapping_order);
+
 #ifdef CONFIG_KVM_GENERIC_GMEM_POPULATE
 long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long npages,
 		       kvm_gmem_populate_cb post_populate, void *opaque)
-- 
2.50.0.727.gbf7dc18ff4-goog