On Fri, Nov 10, 2023, Xiaoyao Li wrote:

On 11/6/2023 12:30 AM, Paolo Bonzini wrote:

quoted

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 68a144cb7dbc..a6de526c0426 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -589,8 +589,20 @@ struct kvm_memory_slot {
  	u32 flags;
  	short id;
  	u16 as_id;
+
+#ifdef CONFIG_KVM_PRIVATE_MEM
+	struct {
+		struct file __rcu *file;
+		pgoff_t pgoff;
+	} gmem;
+#endif
  };
+static inline bool kvm_slot_can_be_private(const struct kvm_memory_slot *slot)
+{
+	return slot && (slot->flags & KVM_MEM_GUEST_MEMFD);
+}
+

maybe we can move this block and ...

<snip>

quoted

@@ -2355,6 +2379,30 @@ bool kvm_arch_pre_set_memory_attributes(struct kvm *kvm,
  					struct kvm_gfn_range *range);
  bool kvm_arch_post_set_memory_attributes(struct kvm *kvm,
  					 struct kvm_gfn_range *range);
+
+static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
+{
+	return IS_ENABLED(CONFIG_KVM_PRIVATE_MEM) &&
+	       kvm_get_memory_attributes(kvm, gfn) & KVM_MEMORY_ATTRIBUTE_PRIVATE;
+}
+#else
+static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
+{
+	return false;
+}
  #endif /* CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES */

this block to Patch 18?

It would work, but my vote is to keep them here to minimize the changes to common
KVM code in the x86 enabling.  It's not a strong preference though.  Of course,
at this point, fiddling with this sort of thing is probably a bad idea in terms
of landing guest_memfd.

quoted

@@ -4844,6 +4875,10 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
  #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
  	case KVM_CAP_MEMORY_ATTRIBUTES:
  		return kvm_supported_mem_attributes(kvm);
+#endif
+#ifdef CONFIG_KVM_PRIVATE_MEM
+	case KVM_CAP_GUEST_MEMFD:
+		return !kvm || kvm_arch_has_private_mem(kvm);
  #endif
  	default:
  		break;

@@ -5277,6 +5312,18 @@ static long kvm_vm_ioctl(struct file *filp,
  	case KVM_GET_STATS_FD:
  		r = kvm_vm_ioctl_get_stats_fd(kvm);
  		break;
+#ifdef CONFIG_KVM_PRIVATE_MEM
+	case KVM_CREATE_GUEST_MEMFD: {
+		struct kvm_create_guest_memfd guest_memfd;

Do we need a guard of below?

		r = -EINVAL;
		if (!kvm_arch_has_private_mem(kvm))
			goto out;

Argh, yeah, that's weird since KVM_CAP_GUEST_MEMFD says "not supported" if the
VM doesn't support private memory.

Enforcing that would break guest_memfd_test.c though.  And having to create a
"special" VM just to test basic guest_memfd functionality would be quite
annoying.

So my vote is to do:

	case KVM_CAP_GUEST_MEMFD:
		return IS_ENABLED(CONFIG_KVM_PRIVATE_MEM);

There's no harm to KVM if userspace creates a file it can't use, and at some
point KVM will hopefully support guest_memfd irrespective of private memory.