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 <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-30-qperret@google.com

1 files changed, 20 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 4ae19247837b..eea2e2b0acaa 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -239,6 +239,26 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
 			   void *mc);
 
 /**
+ * kvm_pgtable_stage2_set_owner() - Unmap and annotate pages in the IPA space to
+ *				    track ownership.
+ * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init().
+ * @addr:	Base intermediate physical address to annotate.
+ * @size:	Size of the annotated range.
+ * @mc:		Cache of pre-allocated and zeroed memory from which to allocate
+ *		page-table pages.
+ * @owner_id:	Unique identifier for the owner of the page.
+ *
+ * By default, all page-tables are owned by identifier 0. This function can be
+ * used to mark portions of the IPA space as owned by other entities. When a
+ * stage 2 is used with identity-mappings, these annotations allow to use the
+ * page-table data structure as a simple rmap.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
+				 void *mc, u8 owner_id);
+
+/**
  * kvm_pgtable_stage2_unmap() - Remove a mapping from a guest stage-2 page-table.
  * @pgt:	Page-table structure initialised by kvm_pgtable_stage2_init().
  * @addr:	Intermediate physical address from which to remove the mapping.