[PATCH v8 24/46] KVM: guest_memfd: Make in-place conversion the default

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Make in-place conversion the default if the arch has private mem.

The default can be overridden at compile type by enabling
CONFIG_KVM_VM_MEMORY_ATTRIBUTES, or at KVM load time through a module
parameter.

In-place conversion also implies tracking a guest's private/shared state in
guest_memfd. To avoid inconsistencies in the way memory attributes are
tracked between the per-VM or by guest_memfd, make the module_param
read-only (0444).

Document that using per-VM attributes for tracking private/shared state of
guest memory is deprecated in favor of tracking in guest_memfd.

Warn if the admin sets gmem_in_place_conversion as false when
CONFIG_KVM_VM_MEMORY_ATTRIBUTES is not enabled. Add warning in the code
path where guest memory is populated for a CoCo VM, since that's the
earliest point in a CoCo VM's lifecycle where memory attributes are
queried. Unlike other query sites, this site is exclusively used by CoCo
VMs.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/Kconfig   | 7 ++++++-
 virt/kvm/guest_memfd.c | 5 +++++
 virt/kvm/kvm_main.c    | 3 ++-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index c28393dc664eb..a3c189d765150 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -85,7 +85,12 @@ config KVM_VM_MEMORY_ATTRIBUTES
 	bool "Enable per-VM PRIVATE vs. SHARED attributes (for CoCo VMs)"
 	help
 	  Enable support for tracking PRIVATE vs. SHARED memory using per-VM
-	  memory attributes.
+	  memory attributes.  Using per-VM attributes are deprecated in favor
+	  of tracking PRIVATE state in guest_memfd.  Select this if you need
+	  to run CoCo VMs using a VMM that doesn't support guest_memfd memory
+	  attributes.
+
+	  If unsure, say N.
 
 config KVM_SW_PROTECTED_VM
 	bool "Enable support for KVM software-protected VMs"
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 86c9f5b0863cb..5cb73543c03c8 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -1193,10 +1193,15 @@ static bool kvm_gmem_range_is_private(struct file *file, pgoff_t index,
 {
 	struct maple_tree *mt = &GMEM_I(file_inode(file))->attributes;
 
+#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
 	if (!gmem_in_place_conversion)
 		return kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + nr_pages,
 							  KVM_MEMORY_ATTRIBUTE_PRIVATE,
 							  KVM_MEMORY_ATTRIBUTE_PRIVATE);
+#else
+	if (WARN_ON_ONCE(!gmem_in_place_conversion))
+		return false;
+#endif
 
 	return kvm_gmem_range_has_attributes(mt, index, nr_pages,
 					     KVM_MEMORY_ATTRIBUTE_PRIVATE);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dd1d18a1d2f68..46e92b5dc3804 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -102,7 +102,8 @@ static bool __ro_after_init allow_unsafe_mappings;
 module_param(allow_unsafe_mappings, bool, 0444);
 
 #ifdef kvm_arch_has_private_mem
-bool __ro_after_init gmem_in_place_conversion = false;
+bool __ro_after_init gmem_in_place_conversion = !IS_ENABLED(CONFIG_KVM_VM_MEMORY_ATTRIBUTES);
+module_param(gmem_in_place_conversion, bool, 0444);
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(gmem_in_place_conversion);
 #endif
 

-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog