[PATCH 08/14] KVM: arm64: Add support for tagging shared pages in page-table

[Quentin Perret]

The hypervisor will soon be in charge of tracking ownership of all
memory pages in the system. The current page-tracking infrastructure at
EL2 only allows binary states: a page is either owned or not by an
entity. But a number of use-cases will require more complex states for
pages that are shared between two entities (host, hypervisor, or guests).

In preparation for supporting these use-cases, introduce in the KVM
page-table library some infrastructure allowing to tag shared pages
using ignored bits (a.k.a. software bits) in PTEs.

Signed-off-by: Quentin Perret <qperret at google.com>
---
 arch/arm64/include/asm/kvm_pgtable.h |  5 +++++
 arch/arm64/kvm/hyp/pgtable.c         | 25 +++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index dd72653314c7..f6d3d5c8910d 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -81,6 +81,8 @@ enum kvm_pgtable_stage2_flags {
  * @KVM_PGTABLE_PROT_W:		Write permission.
  * @KVM_PGTABLE_PROT_R:		Read permission.
  * @KVM_PGTABLE_PROT_DEVICE:	Device attributes.
+ * @KVM_PGTABLE_STATE_SHARED:	Page shared with another entity.
+ * @KVM_PGTABLE_STATE_BORROWED:	Page borrowed from another entity.
  */
 enum kvm_pgtable_prot {
 	KVM_PGTABLE_PROT_X			= BIT(0),
@@ -88,6 +90,9 @@ enum kvm_pgtable_prot {
 	KVM_PGTABLE_PROT_R			= BIT(2),
 
 	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
+
+	KVM_PGTABLE_STATE_SHARED		= BIT(4),
+	KVM_PGTABLE_STATE_BORROWED		= BIT(5),
 };
 
 #define KVM_PGTABLE_PROT_RW	(KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 5bdbe7a31551..51598b79dafc 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -211,6 +211,29 @@ static kvm_pte_t kvm_init_invalid_leaf_owner(u8 owner_id)
 	return FIELD_PREP(KVM_INVALID_PTE_OWNER_MASK, owner_id);
 }
 
+static kvm_pte_t pte_ignored_bit_prot(enum kvm_pgtable_prot prot)
+{
+	kvm_pte_t ignored_bits = 0;
+
+	/*
+	 * Ignored bits 0 and 1 are reserved to track the memory ownership
+	 * state of each page:
+	 *   00: The page is owned solely by the page-table owner.
+	 *   01: The page is owned by the page-table owner, but is shared
+	 *       with another entity.
+	 *   10: The page is shared with, but not owned by the page-table owner.
+	 *   11: Reserved for future use (lending).
+	 */
+	if (prot & KVM_PGTABLE_STATE_SHARED) {
+		if (prot & KVM_PGTABLE_STATE_BORROWED)
+			ignored_bits |= BIT(1);
+		else
+			ignored_bits |= BIT(0);
+	}
+
+	return FIELD_PREP(KVM_PTE_LEAF_ATTR_IGNORED, ignored_bits);
+}
+
 static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
 				  u32 level, kvm_pte_t *ptep,
 				  enum kvm_pgtable_walk_flags flag)
@@ -357,6 +380,7 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
 	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_AP, ap);
 	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_SH, sh);
 	attr |= KVM_PTE_LEAF_ATTR_LO_S1_AF;
+	attr |= pte_ignored_bit_prot(prot);
 	*ptep = attr;
 
 	return 0;
@@ -558,6 +582,7 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
 
 	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
 	attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
+	attr |= pte_ignored_bit_prot(prot);
 	*ptep = attr;
 
 	return 0;
-- 
2.32.0.402.g57bb445576-goog