[PATCH 06/13] KVM: Disallow hugepages for incompatible gmem bindings, but let 'em succeed

[Sean Christopherson]

Remove the restriction that a guest_memfd instance that supports hugepages
can *only* be bound by memslots that are 100% compatible with hugepage
mappings, and instead force KVM to use an order-0 mapping if the binding
isn't compatible with hugepages.

The intent of the draconian binding restriction was purely to simplify the
guest_memfd implementation, e.g. to avoid repeatining the existing logic in
KVM x86ial for precisely tracking which GFNs support hugepages.  But
checking that the binding's offset and size is compatible is just as easy
to do when KVM wants to create a mapping.

And on the other hand, completely rejecting bindings that are incompatible
with hugepages makes it practically impossible for userspace to use a
single guest_memfd instance for all guest memory, e.g. on x86 it would be
impossible to skip the legacy VGA hole while still allowing hugepage
mappings for the rest of guest memory.

Suggested-by: Michael Roth <michael.roth@xxxxxxx>
Link: https://lore.kernel.org/all/20230918163647.m6bjgwusc7ww5tyu@xxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 virt/kvm/guest_mem.c | 54 ++++++++++++++++++++++----------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
index 68528e9cddd7..4f3a313f5532 100644
--- a/virt/kvm/guest_mem.c
+++ b/virt/kvm/guest_mem.c
@@ -434,20 +434,6 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags,
 	return err;
 }
 
-static bool kvm_gmem_is_valid_size(loff_t size, u64 flags)
-{
-	if (size < 0 || !PAGE_ALIGNED(size))
-		return false;
-
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
-	    !IS_ALIGNED(size, HPAGE_PMD_SIZE))
-		return false;
-#endif
-
-	return true;
-}
-
 int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 {
 	loff_t size = args->size;
@@ -460,9 +446,15 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	if (flags & ~valid_flags)
 		return -EINVAL;
 
-	if (!kvm_gmem_is_valid_size(size, flags))
+	if (size < 0 || !PAGE_ALIGNED(size))
 		return -EINVAL;
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
+	    !IS_ALIGNED(size, HPAGE_PMD_SIZE))
+		return -EINVAL;
+#endif
+
 	return __kvm_gmem_create(kvm, size, flags, kvm_gmem_mnt);
 }
 
@@ -470,7 +462,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 		  unsigned int fd, loff_t offset)
 {
 	loff_t size = slot->npages << PAGE_SHIFT;
-	unsigned long start, end, flags;
+	unsigned long start, end;
 	struct kvm_gmem *gmem;
 	struct inode *inode;
 	struct file *file;
@@ -489,16 +481,9 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 		goto err;
 
 	inode = file_inode(file);
-	flags = (unsigned long)inode->i_private;
 
-	/*
-	 * For simplicity, require the offset into the file and the size of the
-	 * memslot to be aligned to the largest possible page size used to back
-	 * the file (same as the size of the file itself).
-	 */
-	if (!kvm_gmem_is_valid_size(offset, flags) ||
-	    !kvm_gmem_is_valid_size(size, flags))
-		goto err;
+	if (offset < 0 || !PAGE_ALIGNED(offset))
+		return -EINVAL;
 
 	if (offset + size > i_size_read(inode))
 		goto err;
@@ -599,8 +584,23 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	page = folio_file_page(folio, index);
 
 	*pfn = page_to_pfn(page);
-	if (max_order)
-		*max_order = compound_order(compound_head(page));
+	if (!max_order)
+		goto success;
+
+	*max_order = compound_order(compound_head(page));
+	if (!*max_order)
+		goto success;
+
+	/*
+	 * For simplicity, allow mapping a hugepage if and only if the entire
+	 * binding is compatible, i.e. don't bother supporting mapping interior
+	 * sub-ranges with hugepages (unless userspace comes up with a *really*
+	 * strong use case for needing hugepages within unaligned bindings).
+	 */
+	if (!IS_ALIGNED(slot->gmem.pgoff, 1ull << *max_order) ||
+	    !IS_ALIGNED(slot->npages, 1ull << *max_order))
+		*max_order = 0;
+success:
 	r = 0;
 
 out_unlock:
-- 
2.42.0.515.g380fc7ccd1-goog