On Mon, Jun 22, 2026, Yan Zhao wrote:

On Thu, Jun 18, 2026 at 05:32:00PM -0700, Ackerley Tng via B4 Relay wrote:

quoted

From: Ackerley Tng <redacted>

Update tdx_gmem_post_populate() to handle cases where a source page is
not explicitly provided. Instead of returning -EOPNOTSUPP when src_page
is NULL, default to using the page associated with the destination PFN.

This change allows for in-place memory conversion where the data is
already present in the target PFN, ensuring the TDX module has a valid
source page reference for the TDH.MEM.PAGE.ADD operation.

Signed-off-by: Ackerley Tng <redacted>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 Documentation/virt/kvm/x86/intel-tdx.rst |  4 ++++
 arch/x86/kvm/vmx/tdx.c                   | 11 ++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/Documentation/virt/kvm/x86/intel-tdx.rst b/Documentation/virt/kvm/x86/intel-tdx.rst
index 6a222e9d09541..74357fe87f9ec 100644
--- a/Documentation/virt/kvm/x86/intel-tdx.rst
+++ b/Documentation/virt/kvm/x86/intel-tdx.rst

@@ -158,6 +158,10 @@ KVM_TDX_INIT_MEM_REGION
 Initialize @nr_pages TDX guest private memory starting from @gpa with userspace
 provided data from @source_addr. @source_addr must be PAGE_SIZE-aligned.
 
+If guest_memfd in-place conversion is enabled, pass NULL for @source_addr to
+initialize the memory region using memory contents already populated in
+guest_memfd memory.
+
 Note, before calling this sub command, memory attribute of the range
 [gpa, gpa + nr_pages] needs to be private.  Userspace can use
 KVM_SET_MEMORY_ATTRIBUTES to set the attribute.

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index ffe9d0db58c59..56d10333c61a7 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c

@@ -3198,8 +3198,12 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	if (KVM_BUG_ON(kvm_tdx->page_add_src, kvm))
 		return -EIO;
 
-	if (!src_page)
-		return -EOPNOTSUPP;
+	if (!src_page) {
+		if (!gmem_in_place_conversion)

When userspace turns on gmem_in_place_conversion while creating guest_memfd
without the MMAP flag, the absence of src_page should still be treated as an
error.

Why MMAP?  Shouldn't this be a general "if (!src_page && !up-to-date)"?  Just
because userspace _can_ mmap() the memory doesn't mean userspace _has_ mmap()'d
and written memory.  And when write() lands, MMAP wouldn't be necessary to
initialize the memory.

Additionally, to properly enable in-place copying for the TDX initial memory
region, userspace must not only specify source_addr to NULL, but also follow
a specific sequence (where steps 1/2/3/7 are required only for in-place copy):
1. create guest_memfd with MMAP flag
2. mmap the guest_memfd.
3. convert the initial memory range to shared.
4. copy initial content to the source page.
5. convert the initial memory range to private
6. invoke ioctl KVM_TDX_INIT_MEM_REGION.
7. do not unmap the source backend.

So, would it be reasonable to introduce a dedicated flag that allows userspace
to explicitly opt into the in-place copy functionality? e.g.,

Why?  It's userspace's responsibility to get the above right.  If userspace fails
to provide a src_page when it doesn't want in-place copy, that's a userspace bug.