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

[Sean Christopherson]

On Fri, Jul 31, 2026, Xiaoyao Li wrote:
> 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(&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?

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.

[*] https://lore.kernel.org/all/akMPZePBdwQlD74H@xxxxxxxxxx