[PATCH v8 25/46] KVM: guest_memfd: Enable INIT_SHARED on guest_memfd for x86 Coco VMs

[Ackerley Tng via B4 Relay]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Now that guest_memfd supports tracking private vs. shared within gmem
itself, allow userspace to specify INIT_SHARED on a guest_memfd instance
for x86 Confidential Computing (CoCo) VMs, so long as in-place conversion
is enabled, i.e. when it's actually possible for a guest_memfd instance to
contain shared memory.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2fde594e86d72..57a543dadb851 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -14116,14 +14116,15 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
 }
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
-/*
- * KVM doesn't yet support initializing guest_memfd memory as shared for VMs
- * with private memory (the private vs. shared tracking needs to be moved into
- * guest_memfd).
- */
 bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
 {
-	return !kvm_arch_has_private_mem(kvm);
+	/*
+	 * INIT_SHARED is supported if in-place conversion is enabled, or if
+	 * the VM doesn't support private memory.  If the VM has private memory
+	 * and in-place conversion is disabled, then guest_memfd can _only_ be
+	 * used for private memory.
+	 */
+	return gmem_in_place_conversion || !kvm_arch_has_private_mem(kvm);
 }
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE

-- 
2.55.0.rc0.738.g0c8ab3ebcc-goog