From: Sean Christopherson <seanjc@google.com>

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 per-VM attributes
are disabled, i.e. when it's actually possible for a guest_memfd instance
to contain shared memory.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b2e93f836dca..6518cdb4569f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13984,14 +13984,13 @@ 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 isn't supported if the memory attributes are per-VM,
+	 * in which case guest_memfd can _only_ be used for private memory.
+	 */
+	return !vm_memory_attributes || !kvm_arch_has_private_mem(kvm);
 }
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
-- 
2.53.0.rc1.225.gd81095ad13-goog