Re: [PATCH v5 29/36] KVM: arm64: Use page-table to track page ownership

As the host stage 2 will be identity mapped, all the .hyp memory regions
and/or memory pages donated to protected guestis will have to marked
invalid in the host stage 2 page-table. At the same time, the hypervisor
will need a way to track the ownership of each physical page to ensure
memory sharing or donation between entities (host, guests, hypervisor) is
legal.

In order to enable this tracking at EL2, let's use the host stage 2
page-table itself. The idea is to use the top bits of invalid mappings
to store the unique identifier of the page owner. The page-table owner
(the host) gets identifier 0 such that, at boot time, it owns the entire
IPA space as the pgd starts zeroed.

Provide kvm_pgtable_stage2_set_owner() which allows to modify the
ownership of pages in the host stage 2. It re-uses most of the map()
logic, but ends up creating invalid mappings instead. This impacts
how we do refcount as we now need to count invalid mappings when they
are used for ownership tracking.

Signed-off-by: Quentin Perret <redacted>
---
 arch/arm64/include/asm/kvm_pgtable.h |  21 +++++
 arch/arm64/kvm/hyp/pgtable.c         | 127 ++++++++++++++++++++++-----
 2 files changed, 124 insertions(+), 24 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 4ae19247837b..683e96abdc24 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h

@@ -238,6 +238,27 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
 			   u64 phys, enum kvm_pgtable_prot prot,
 			   void *mc);
 
+/**
+ * kvm_pgtable_stage2_set_owner() - Annotate invalid mappings with metadata
+ *				    encoding the ownership of a page in the
+ *				    IPA space.

+int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
+				 void *mc, u8 owner_id)
+{
+	int ret;
+	struct stage2_map_data map_data = {
+		.phys		= KVM_PHYS_INVALID,
+		.mmu		= pgt->mmu,
+		.memcache	= mc,
+		.mm_ops		= pgt->mm_ops,
+		.owner_id	= owner_id,
+	};
+	struct kvm_pgtable_walker walker = {
+		.cb		= stage2_map_walker,
+		.flags		= KVM_PGTABLE_WALK_TABLE_PRE |
+				  KVM_PGTABLE_WALK_LEAF |
+				  KVM_PGTABLE_WALK_TABLE_POST,
+		.arg		= &map_data,
+	};
+
+	if (owner_id > KVM_MAX_OWNER_ID)
+		return -EINVAL;
+
+	ret = kvm_pgtable_walk(pgt, addr, size, &walker);
+	dsb(ishst);