Re: [PATCH v10 12/41] KVM: guest_memfd: Call arch make_shared callback for to-shared conversion

Sean Christopherson [off-list ref] writes:

On Thu, Aug 13, 2026, Ackerley Tng wrote:

quoted

Sean Christopherson [off-list ref] writes:

quoted

On Thu, Aug 13, 2026, Binbin Wu wrote:

quoted

On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:

quoted

+#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
+static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start, pgoff_t end)
+{
+	struct folio_batch fbatch;
+	pgoff_t next = start;
+	int i;
+
+	folio_batch_init(&fbatch);
+	while (filemap_get_folios(inode->i_mapping, &next, end - 1, &fbatch)) {
+		for (i = 0; i < folio_batch_count(&fbatch); ++i) {
+			struct folio *folio = fbatch.folios[i];
+			pgoff_t start_index, end_index;
+			kvm_pfn_t start_pfn;
+			kvm_pfn_t nr_pages;
+
+			start_index = max(start, folio->index);
+			end_index = min(end, folio_next_index(folio));
+			/*
+			 * end_index is either in folio or points to
+			 * the first page of the next folio. Hence,
+			 * all pages in range [start_index, end_index)
+			 * are contiguous.
+			 */
+			start_pfn = folio_file_pfn(folio, start_index);
+			nr_pages = end_index - start_index;
+
+			kvm_arch_gmem_make_shared(start_pfn, nr_pages);
+		}
+
+		folio_batch_release(&fbatch);
+		cond_resched();
+	}
+}
+#else
+static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start, pgoff_t end) {}
+#endif
+
 static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
 				     size_t nr_pages, uint64_t attrs,
 				     pgoff_t *err_index)

@@ -599,7 +636,12 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,

 	filter = to_private ? KVM_FILTER_SHARED : KVM_FILTER_PRIVATE;
 	kvm_gmem_invalidate_start(inode, start, end, filter);
+
+	if (!to_private)

Should I make this condition

	if (!to_private && kvm_x86_ops.gmem_make_shared)

instead?

Will that help?

No, because that's bleeding x86 details into common code, which defeats the purpose
of arch hooks.

Ah okay yup maybe it should have been kvm_arch_has_gmem_make_shared()
that defaults to false and x86 will return !!kvm_x86_ops.gmem_make_shared.

That's why I think it's worth analyzing the cost: if it's in the
noise, leave it alone.  If it's meaningful, figure out a not-too-gross way to skip
the entire thing if kvm_arch_gmem_make_shared() is a glorified nop in the end.

Is noise defined relative to the entire conversion process? Would this
benchmark look like

1. Convert 4G to shared on TDX with CONFIG_AMD_SEV defined
2. Convert 4G to shared on TDX without CONFIG_AMD_SEV defined

and then compare the difference in time taken?