Re: [PATCH v12 23/45] KVM: Let userspace disable per-VM mem attributes, enable per-gmem attributes

From: Sean Christopherson <seanjc@google.com>

Allow the user to disable KVM_VM_MEMORY_ATTRIBUTES even when KVM supports
PRIVATE and SHARED attributes, and expose gmem_in_place_conversion as a
module parameter when per-VM attributes are supported.  I.e. let userspace
enable in-place PRIVATE<=>SHARED conversion of guest_memfd pages.

Provide both a Kconfig option and a (conditional) module param so that
deployments that use a custom kernel can fully disable per-VM tracking,
while not forcing distros to ship two separate kernels in order to provide
backwards compatibility for downstream users.

Don't allow running VMs with mixed tracking for a given instance of KVM,
i.e. disallow toggling the module param after KVM is loaded, as the extra
complexity needed to handle per-VM behavior far outweighs any potential
benefit.  E.g. neither TDX nor SNP supports live migration, so in effect
the requirement is that existing deployments that want to support both the
old and the new models would need to tell their VMM which flavor of
tracking to use.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Fuad Tabba <redacted>
Tested-by: Shivank Garg <redacted>
[Define module_param only if CONFIG_KVM_VM_MEMORY_ATTRIBUTES is enabled]
Suggested-by: Xiaoyao Li <redacted>
Reviewed-by: Xiaoyao Li <redacted>
Co-developed-by: Ackerley Tng <redacted>
Signed-off-by: Ackerley Tng <redacted>
---
 Documentation/admin-guide/kernel-parameters.txt | 25 +++++++++++++++++++++++++
 arch/x86/include/asm/kvm_host.h                 |  4 +++-
 arch/x86/kvm/Kconfig                            | 14 ++++++++++----
 virt/kvm/kvm_main.c                             |  5 ++++-
 4 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 68647ff4bdd24..b5719714458fb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -3156,6 +3156,31 @@ Kernel parameters
 	kvm.enable_vmware_backdoor=[KVM] Support VMware backdoor PV interface.
 				   Default is false (don't support).
 
+	kvm.gmem_in_place_conversion=
+			[KVM] Controls whether KVM enables in-place conversion
+			support for guest_memfd and tracks the private/shared
+			state of memory per guest_memfd instead of per VM.
+
+			If enabled, KVM enables the KVM_SET_MEMORY_ATTRIBUTES2
+			ioctl on guest_memfd file descriptors and disables the
+			legacy VM-scoped KVM_SET_MEMORY_ATTRIBUTES ioctl for
+			private memory state tracking. Only the
+			KVM_MEMORY_ATTRIBUTE_PRIVATE attribute moves to
+			per-guest_memfd tracking; other attributes remain
+			per-VM.
+
+			This parameter toggles KVM's in-place conversion
+			capability support. Whether a VMM uses separate backends
+			or out-of-place memory management is determined by
+			userspace VMM design.
+
+			Note, this parameter is only available when
+			CONFIG_KVM_VM_MEMORY_ATTRIBUTES=y. When
+			CONFIG_KVM_VM_MEMORY_ATTRIBUTES is not set, in-place
+			conversion is unconditionally enabled.
+
+			Default is Y (on).