Re: [PATCH v9 18/41] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@google.com>
> 
> 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@google.com>
> Tested-by: Shivank Garg <shivankg@amd.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---
>   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(&region, 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@yzhao56-desk.sh.intel.com/

>   	    !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--;
>   
>