Re: [PATCH v9 18/41] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION
From: Xiaoyao Li
Date: Fri Jul 31 2026 - 04:32:11 EST
On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
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 <seanjc@xxxxxxxxxx>
Tested-by: Shivank Garg <shivankg@xxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
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?
[1] https://lore.kernel.org/all/akI9m02jgKAdi4gX@xxxxxxxxxxxxxxxxxxxxxxxxx/
!PAGE_ALIGNED(region.gpa) || !region.nr_pages ||
region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa ||
!vt_is_tdx_private_gpa(kvm, region.gpa) ||
@@ -3269,7 +3270,8 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
break;
}
- region.source_addr += PAGE_SIZE;
+ if (region.source_addr)
+ region.source_addr += PAGE_SIZE;
region.gpa += PAGE_SIZE;
region.nr_pages--;