[PATCH v12 16/45] KVM: guest_memfd: Call arch make_shared callback for to-shared conversion

From: Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>
Date: 2026-08-31 00:25:21
Also in: b4-sent, kvm, linux-coco, linux-doc, linux-kselftest, linux-mm, lkml
Reply to: Ackerley Tng via B4 Relay, 2026-08-31

From: Ackerley Tng <redacted>

When doing in-place conversion from PRIVATE to SHARED, immediately inform
arch code of the conversion for all allocated pages/folios, e.g. so that
arch code can put hardware metadata tables in the correct state.  Eagerly
updating the table for to SHARED conversions avoids having to implement
on-demand updates, e.g. when faulting in host userspace mappings.  Skip the
entire flow if the arch doesn't implement conversion callbacks, as getting
folios from the filemap is noticeably expensive, especially when converting
large chunks of memory.

Deliberately don't eagerly update the metadata table on conversions from
SHARED to PRIVATE, because assigning a page to a VM (versus "returning" it
to the host) requires the exact GFN associated with the page, i.e would
require walking the memslot bindings.  And because KVM *must* do on-demand
metadata updates when getting a PFN for KVM-internal usage, as that's the
only time a relevant memslot binding is guaranteed to exist.

Note!  Inform arch code of the conversion within the protection of the
invalidation sequence, to ensure that any existing mappings are dropped
before hardware is updated, and to ensure that new mappings can't be
established until after the conversion is complete.

Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Signed-off-by: Ackerley Tng <redacted>
---
 arch/x86/include/asm/kvm-x86-ops.h |  2 +-
 arch/x86/include/asm/kvm_host.h    |  2 +-
 arch/x86/kvm/x86.c                 |  5 +++++
 include/linux/kvm_host.h           |  1 +
 virt/kvm/guest_memfd.c             | 42 ++++++++++++++++++++++++++++++++++++++
 5 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
index e213c9ae3e301..67b43c167045b 100644
--- a/arch/x86/include/asm/kvm-x86-ops.h
+++ b/arch/x86/include/asm/kvm-x86-ops.h

@@ -150,7 +150,7 @@ KVM_X86_OP_OPTIONAL(alloc_apic_backing_page)
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
 KVM_X86_OP_OPTIONAL_RET0(gmem_make_private)
 #endif
-#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM
+#if defined(CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT) || defined(CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM)
 KVM_X86_OP_OPTIONAL(gmem_make_shared)
 #endif
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 744c1f6ff03ed..83e26ce45fb79 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h

@@ -1732,7 +1732,7 @@ struct kvm_x86_ops {
 	int (*gmem_make_private)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 				 kvm_pfn_t nr_pages);
 #endif
-#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM
+#if defined(CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT) || defined(CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM)
 	void (*gmem_make_shared)(kvm_pfn_t pfn, kvm_pfn_t nr_pages);
 #endif
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_INVALIDATE

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2292249570314..75e03a2f79db2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c

@@ -10653,6 +10653,11 @@ int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 {
 	return kvm_x86_call(gmem_make_private)(kvm, gfn, pfn, nr_pages);
 }
+
+void kvm_arch_gmem_make_shared(kvm_pfn_t pfn, kvm_pfn_t nr_pages)
+{
+	kvm_x86_call(gmem_make_shared)(pfn, nr_pages);
+}
 #endif
 
 #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ab87effdd221f..485f18454eb45 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -2610,6 +2610,7 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 
 int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 			       kvm_pfn_t nr_pages);
+void kvm_arch_gmem_make_shared(kvm_pfn_t pfn, kvm_pfn_t nr_pages);
 #ifndef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
 #define kvm_arch_has_gmem_convert() false
 #endif

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index fe02c47c85fb5..d14a7024bdc7b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c

@@ -575,6 +575,43 @@ static bool kvm_gmem_has_outstanding_references(struct inode *inode,
 	return has_outstanding;
 }
 
+#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)

@@ -624,7 +661,12 @@ 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 && kvm_arch_has_gmem_convert())
+		kvm_gmem_make_shared(inode, start, end);
+
 	mas_store_prealloc(&mas, xa_mk_value(attrs));
+
 	kvm_gmem_invalidate_end(inode, start, end);
 out:
 	filemap_invalidate_unlock(mapping);

-- 
2.55.0.897.gb25b4bd76c-goog

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