On 8/1/2026 12:11 AM, Sean Christopherson wrote:
On Fri, Jul 31, 2026, Xiaoyao Li wrote:
On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
From: Ackerley Tng <[email protected]>
Update tdx_gmem_post_populate() to handle cases where userspace requests
"no source page". To handle "no source page", populate (perform
TDH.MEM.PAGE.ADD) using memory in-place at the target PFN.
Signed-off-by: Sean Christopherson <[email protected]>
Tested-by: Shivank Garg <[email protected]>
Signed-off-by: Ackerley Tng <[email protected]>
---
Documentation/virt/kvm/x86/intel-tdx.rst | 4 ++++
arch/x86/kvm/vmx/tdx.c | 8 +++++---
2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/Documentation/virt/kvm/x86/intel-tdx.rst
b/Documentation/virt/kvm/x86/intel-tdx.rst
index 6a222e9d09541..d8d9409120e61 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 0 for @source_addr
+to represent "no source page". A source page is required if in-place
+conversion is not enabled or not supported.
+
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 d1af0a752e97e..e890da5f18aed 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3192,7 +3192,7 @@ 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;
- kvm_tdx->page_add_src = src_page;
+ kvm_tdx->page_add_src = src_page ?: pfn_to_page(pfn);
ret = kvm_tdp_mmu_map_private_pfn(arg->vcpu, gfn, pfn);
kvm_tdx->page_add_src = NULL;
@@ -3238,7 +3238,8 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu
*vcpu, struct kvm_tdx_cmd *c
if (copy_from_user(®ion, u64_to_user_ptr(cmd->data), sizeof(region)))
return -EFAULT;
- if (!PAGE_ALIGNED(region.source_addr) || !region.source_addr ||
+ if (!PAGE_ALIGNED(region.source_addr) ||
+ (!gmem_in_place_conversion && !region.source_addr) ||
Sorry, I still want to discuss why we only allow in-place PAGE.ADD for
gmem_in_place_conversion only. Though Yan raised this opinion[1] in previous
v8, I'm not quite following her argument. So let me try again.
I think in-place PAGE.ADD of TDX doesn't need to depend on
gmem_in_place_conversion. There are two use cases actually.
1). Userspace sets the gmem page as private, and invokes the in-place
PAGE.ADD by passing a 0 source_addr. Due to patch 16, the target PFN will be
ADD'ed to TD as a all-0 page.
2). Userspace sets the gmem page as shared, and writes the desired content
to it. Then converts the page to private, and invokes the in-place PAGE.ADD
by passing a 0 source_addr. The target PFN will be ADD'ed to TD with desired
content.
For gmem_in_place_conversion == false, only case 1) is possible.
For gmem_in_place_conversion == true, both case 1) and 2) are possible.
Basically, this patch is changing the behavior of KVM_TDX_INIT_MEM_REGION.
Before, it returns -EINVAL when region.source_addr == 0. Now it wants to
allow the "region.source_addr == 0" case. If we can allow it
unconditionally, why bother adding the restriction to allow it only when
gmem_in_place_conversion == true?
As I said[*] in that thread:
: Because retroactively adding support for out-of-place conversion is
pointless
: (requires a userspace update for a feature that's being deprecated), KVM
can't
: generally support using the source for out-of-place conversion (it's
effectively
: an obscure zero-page optimization), and IMO rejecting the out-of-place
conversion
: scenario is valuable for KVM developers, e.g. to help newcomers understand
what
: exactly is and isn't possible.
Fair enough.
Ackerley, please update the changelog to add the reason why we decided
to only support in-place PAGE.ADD for TDX for in-place gmem only.
[*] https://lore.kernel.org/all/[email protected]
