KVM: Assert slots_lock is held when setting memory regions

Add proper lockdep assertions in __kvm_set_memory_region() and
__x86_set_memory_region() instead of relying comments.

Opportunistically delete __kvm_set_memory_region()'s entire function
comment as the API doesn't allocate memory or select a gfn, and the
"mostly for framebuffers" comment hasn't been true for a very long time.

Cc: Tao Su <tao1.su@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Link: https://lore.kernel.org/r/20250111002022.1230573-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -12793,7 +12793,8 @@ void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa,
 	struct kvm_memslots *slots = kvm_memslots(kvm);
 	struct kvm_memory_slot *slot;
 
-	/* Called with kvm->slots_lock held.  */
+	lockdep_assert_held(&kvm->slots_lock);
+
 	if (WARN_ON(id >= KVM_MEM_SLOTS_NUM))
 		return ERR_PTR_USR(-EINVAL);
 

virt/kvm/kvm_main.c

@@ -1926,14 +1926,6 @@ static bool kvm_check_memslot_overlap(struct kvm_memslots *slots, int id,
 	return false;
 }
 
-/*
- * Allocate some memory and give it an address in the guest physical address
- * space.
- *
- * Discontiguous memory is allowed, mostly for framebuffers.
- *
- * Must be called holding kvm->slots_lock for write.
- */
 int __kvm_set_memory_region(struct kvm *kvm,
 			    const struct kvm_userspace_memory_region2 *mem)
 {
@@ -1945,6 +1937,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	int as_id, id;
 	int r;
 
+	lockdep_assert_held(&kvm->slots_lock);
+
 	r = check_memory_region_flags(kvm, mem);
 	if (r)
 		return r;