[PATCH v5 4/7] KVM: guest_memfd: Fold __kvm_gmem_prepare_folio() into its sole caller

[Sean Christopherson <seanjc@google.com>]

Fold __kvm_gmem_prepare_folio() into kvm_gmem_prepare_folio() and extend
the #ifdef guard to cover all of the kvm_gmem_prepare_folio().  Take care
to align the gfn _before_ computing the index, otherwise KVM would end up
with a mismatched gfn+pfn pair for folios larger than order-0 (this detail
was subtly handled by recomputing gfn in the helper using the aligned pfn).

Providing a single-use, ~2 line tail-call helper adds no value.  In fact,
due to having to reconstitute the gfn, *and* effectively hiding the
alignment adjustment for hugepages, the separate helper is very arguably a
net negative.

Opportunistically convert the existing WARN in the prepare flow to a only
fire once, e.g. so that a guest_memfd bug doesn't unintentionally Dos the
kernel by spamming the log.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/guest_memfd.c | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 62c2c157f0c1..f9a1d6d64f6c 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -60,19 +60,6 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
-static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
-				    pgoff_t index, struct folio *folio)
-{
-#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
-	kvm_pfn_t pfn = folio_file_pfn(folio, index);
-	gfn_t gfn = slot->base_gfn + index - slot->gmem.pgoff;
-
-	return kvm_arch_gmem_prepare(kvm, gfn, pfn, folio_order(folio));
-#else
-	return 0;
-#endif
-}
-
 /*
  * Process @folio, which contains @gfn, so that the guest can use it.
  * The folio must be locked and the gfn must be contained in @slot.
@@ -82,6 +69,7 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo
 static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 				  gfn_t gfn, struct folio *folio)
 {
+#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
 	pgoff_t index;
 
 	/*
@@ -97,11 +85,15 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 	 * The order will be passed when creating the guest_memfd, and
 	 * checked when creating memslots.
 	 */
-	WARN_ON(!IS_ALIGNED(slot->gmem.pgoff, folio_nr_pages(folio)));
+	WARN_ON_ONCE(!IS_ALIGNED(slot->gmem.pgoff, folio_nr_pages(folio)));
+	gfn = ALIGN_DOWN(gfn, folio_nr_pages(folio));
 	index = kvm_gmem_get_index(slot, gfn);
-	index = ALIGN_DOWN(index, folio_nr_pages(folio));
 
-	return __kvm_gmem_prepare_folio(kvm, slot, index, folio);
+	return kvm_arch_gmem_prepare(kvm, gfn, folio_file_pfn(folio, index),
+				     folio_order(folio));
+#else
+	return 0;
+#endif
 }
 
 /*
-- 
2.55.0.141.g00534a21ce-goog