Re: [PATCH v10 18/41] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION

On Fri, Aug 07, 2026 at 02:52:57PM -0700, 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.
> 
> Allow "no source page" only when gmem_in_place_conversion is enabled,
> because retroactively adding support for out-of-place conversion would mean
> requiring a userspace update for a feature that's being deprecated.
> 
> Also, KVM supporting "no source page" without in-place conversion would
> effectively be an obscure zero-page optimization that relies on the page
> being zeroed when it is allocated by guest_memfd.
> 
> Rejecting "no source page" without in-place conversion scenario is valuable
> for KVM developers since it helps newcomers understand what exactly is and
> isn't possible.
> 
> 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 b272c20586a74..7654a9f5fa128 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -3189,7 +3189,7 @@ 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;
>  
> @@ -3235,7 +3235,8 @@ 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) ||
>  	    !PAGE_ALIGNED(region.gpa) || !region.nr_pages ||
>  	    region.gpa + (region.nr_pages << PAGE_SHIFT) <= region.gpa ||
>  	    !vt_is_tdx_private_gpa(kvm, region.gpa) ||
> @@ -3266,7 +3267,8 @@ 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--;

Tested the following scenarios:
1.gmem_in_place_conversion == true,
  (1) region.source_addr == NULL.
      ==> TD launched successfully.

  (2) region.source_addr != NULL,
      region.source_addr is not from mmap() of the guest_memfd (double backends)
      ==> TD launched successfully.

  (3) region.source_addr != NULL,
      region.source_addr is from mmap() of the guest_memfd (single backend),
      GPA is private.
      ==> Failed (-EFAULT).

  (4) region.source_addr != NULL,
      region.source_addr is from mmap() of the guest_memfd (single backend),
      GPA is shared.
      ==> Failed (-EINVAL).

2. gmem_in_place_conversion == false,
  (1) region.source_addr == NULL.
      ==> Failed (-EINVAL).

  (2) region.source_addr is not from mmap() of the guest_memfd.
      ==> TD launched successfully.
 

So,
Tested-by: Yan Zhao <[email protected]>
Reviewed-by:  Yan Zhao <[email protected]>