[RFC PATCH v5 42/45] KVM: guest_memfd: Add helpers to get start/end gfns give gmem+slot+pgoff

[Sean Christopherson <seanjc@google.com>]

Add helpers for getting a gfn given a gmem slot+pgoff, and for getting a
gfn given a starting or ending pgoff, i.e. an offset that may be beyond
the range of the memslot binding.  Providing helpers will avoid duplicate
boilerplate code "if" future code also needs to iterate over gfn ranges.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/guest_memfd.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 923c51a3a525..51dbb309188f 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -59,6 +59,21 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+static gfn_t kvm_gmem_get_gfn(struct kvm_memory_slot *slot, pgoff_t pgoff)
+{
+	return slot->base_gfn + pgoff - slot->gmem.pgoff;
+}
+
+static gfn_t kvm_gmem_get_start_gfn(struct kvm_memory_slot *slot, pgoff_t start)
+{
+	return kvm_gmem_get_gfn(slot, max(slot->gmem.pgoff, start));
+}
+
+static gfn_t kvm_gmem_get_end_gfn(struct kvm_memory_slot *slot, pgoff_t end)
+{
+	return kvm_gmem_get_gfn(slot, min(slot->gmem.pgoff + slot->npages, end));
+}
+
 static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 				    pgoff_t index, struct folio *folio)
 {
@@ -167,11 +182,9 @@ static void __kvm_gmem_invalidate_begin(struct gmem_file *f, pgoff_t start,
 	unsigned long index;
 
 	xa_for_each_range(&f->bindings, index, slot, start, end - 1) {
-		pgoff_t pgoff = slot->gmem.pgoff;
-
 		struct kvm_gfn_range gfn_range = {
-			.start = slot->base_gfn + max(pgoff, start) - pgoff,
-			.end = slot->base_gfn + min(pgoff + slot->npages, end) - pgoff,
+			.start = kvm_gmem_get_start_gfn(slot, start),
+			.end = kvm_gmem_get_end_gfn(slot, end),
 			.slot = slot,
 			.may_block = true,
 			.attr_filter = attr_filter,
-- 
2.53.0.rc1.217.geba53bf80e-goog