[PATCH gmem 2/6] KVM: guest_memfd: Only call kvm_arch_gmem_prepare hook if necessary

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It has been reported that the internal workings of
kvm_gmem_prepare_folio() incurs noticeable overhead for large guests
even for platforms where kvm_arch_gmem_prepare() is a no-op.

Provide a new kvm_arch_gmem_prepare_needed() hook so that architectures
that set CONFIG_HAVE_KVM_GMEM_PREPARE can still opt-out of issuing the
kvm_arch_gmem_prepare() callback if the particular KVM instance doesn't
require any sort of special preparation of its gmem pages prior to use.

Link: https://lore.kernel.org/lkml/20240228202906.GB10568@xxxxxxxxxxxxxxxxxxxxx/
Suggested-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
 include/linux/kvm_host.h |  1 +
 virt/kvm/guest_memfd.c   | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2f5074eff958..5b8308b5e4af 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2466,6 +2466,7 @@ static inline int kvm_gmem_undo_get_pfn(struct kvm *kvm,
 int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order);
+bool kvm_arch_gmem_prepare_needed(struct kvm *kvm);
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 74e19170af8a..4ce0056d1149 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -13,6 +13,13 @@ struct kvm_gmem {
 	struct list_head entry;
+bool __weak kvm_arch_gmem_prepare_needed(struct kvm *kvm)
+	return false;
 static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct folio *folio)
@@ -27,6 +34,9 @@ static int kvm_gmem_prepare_folio(struct inode *inode, pgoff_t index, struct fol
 		gfn_t gfn;
 		int rc;
+		if (!kvm_arch_gmem_prepare_needed(kvm))
+			continue;
 		slot = xa_load(&gmem->bindings, index);
 		if (!slot)

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