[PATCH v14 18/21] KVM: arm64: Enable host mapping of shared guest_memfd memory

[Fuad Tabba <tabba@google.com>]

Enable host userspace mmap support for guest_memfd-backed memory on
arm64. This change provides arm64 with the capability to map guest
memory at the host directly from guest_memfd:

* Define kvm_arch_supports_gmem_mmap() for arm64: The
  kvm_arch_supports_gmem_mmap() macro is defined for arm64 to be true if
  CONFIG_KVM_GMEM_SUPPORTS_MMAP is enabled. For existing arm64 KVM VM
  types that support guest_memfd, this enables them to use guest_memfd
  with host userspace mappings. This provides a consistent behavior as
  there are currently no arm64 CoCo VMs that rely on guest_memfd solely
  for private, non-mappable memory. Future arm64 VM types can override
  or restrict this behavior via the kvm_arch_supports_gmem_mmap() hook
  if needed.

* Select CONFIG_KVM_GMEM_SUPPORTS_MMAP in arm64 Kconfig.

* Enforce KVM_MEMSLOT_GMEM_ONLY for guest_memfd on arm64: Checks are
  added to ensure that if guest_memfd is enabled on arm64,
  KVM_GMEM_SUPPORTS_MMAP must also be enabled. This means
  guest_memfd-backed memory slots on arm64 are currently only supported
  if they are intended for shared memory use cases (i.e.,
  kvm_memslot_is_gmem_only() is true). This design reflects the current
  arm64 KVM ecosystem where guest_memfd is primarily being introduced
  for VMs that support shared memory.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/include/asm/kvm_host.h | 4 ++++
 arch/arm64/kvm/Kconfig            | 2 ++
 arch/arm64/kvm/mmu.c              | 7 +++++++
 3 files changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 3e41a880b062..63f7827cfa1b 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1674,5 +1674,9 @@ void compute_fgu(struct kvm *kvm, enum fgt_group_id fgt);
 void get_reg_fixed_bits(struct kvm *kvm, enum vcpu_sysreg reg, u64 *res0, u64 *res1);
 void check_feature_map(void);
 
+#ifdef CONFIG_KVM_GMEM
+#define kvm_arch_supports_gmem(kvm) true
+#define kvm_arch_supports_gmem_mmap(kvm) IS_ENABLED(CONFIG_KVM_GMEM_SUPPORTS_MMAP)
+#endif
 
 #endif /* __ARM64_KVM_HOST_H__ */
diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 713248f240e0..323b46b7c82f 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -37,6 +37,8 @@ menuconfig KVM
 	select HAVE_KVM_VCPU_RUN_PID_CHANGE
 	select SCHED_INFO
 	select GUEST_PERF_EVENTS if PERF_EVENTS
+	select KVM_GMEM
+	select KVM_GMEM_SUPPORTS_MMAP
 	help
 	  Support hosting virtualized guest machines.
 
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 8c82df80a835..85559b8a0845 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2276,6 +2276,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	if ((new->base_gfn + new->npages) > (kvm_phys_size(&kvm->arch.mmu) >> PAGE_SHIFT))
 		return -EFAULT;
 
+	/*
+	 * Only support guest_memfd backed memslots with mappable memory, since
+	 * there aren't any CoCo VMs that support only private memory on arm64.
+	 */
+	if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
+		return -EINVAL;
+
 	hva = new->userspace_addr;
 	reg_end = hva + (new->npages << PAGE_SHIFT);
 
-- 
2.50.0.727.gbf7dc18ff4-goog