Re: [PATCH v8 23/46] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION

[Sean Christopherson <seanjc@google.com>]

On Mon, Jun 22, 2026, Yan Zhao wrote:
> On Thu, Jun 18, 2026 at 05:32:00PM -0700, Ackerley Tng via B4 Relay wrote:
> > From: Ackerley Tng <ackerleytng@google.com>
> > 
> > Update tdx_gmem_post_populate() to handle cases where a source page is
> > not explicitly provided. Instead of returning -EOPNOTSUPP when src_page
> > is NULL, default to using the page associated with the destination PFN.
> > 
> > This change allows for in-place memory conversion where the data is
> > already present in the target PFN, ensuring the TDX module has a valid
> > source page reference for the TDH.MEM.PAGE.ADD operation.
> > 
> > Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  Documentation/virt/kvm/x86/intel-tdx.rst |  4 ++++
> >  arch/x86/kvm/vmx/tdx.c                   | 11 ++++++++---
> >  2 files changed, 12 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/virt/kvm/x86/intel-tdx.rst b/Documentation/virt/kvm/x86/intel-tdx.rst
> > index 6a222e9d09541..74357fe87f9ec 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 NULL for @source_addr to
> > +initialize the memory region using memory contents already populated in
> > +guest_memfd memory.
> > +
> >  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 ffe9d0db58c59..56d10333c61a7 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -3198,8 +3198,12 @@ 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;
> >  
> > -	if (!src_page)
> > -		return -EOPNOTSUPP;
> > +	if (!src_page) {
> > +		if (!gmem_in_place_conversion)
> When userspace turns on gmem_in_place_conversion while creating guest_memfd
> without the MMAP flag, the absence of src_page should still be treated as an
> error.

Why MMAP?  Shouldn't this be a general "if (!src_page && !up-to-date)"?  Just
because userspace _can_ mmap() the memory doesn't mean userspace _has_ mmap()'d
and written memory.  And when write() lands, MMAP wouldn't be necessary to
initialize the memory.

> Additionally, to properly enable in-place copying for the TDX initial memory
> region, userspace must not only specify source_addr to NULL, but also follow
> a specific sequence (where steps 1/2/3/7 are required only for in-place copy):
> 1. create guest_memfd with MMAP flag
> 2. mmap the guest_memfd.
> 3. convert the initial memory range to shared.
> 4. copy initial content to the source page.
> 5. convert the initial memory range to private
> 6. invoke ioctl KVM_TDX_INIT_MEM_REGION.
> 7. do not unmap the source backend.
> 
> So, would it be reasonable to introduce a dedicated flag that allows userspace
> to explicitly opt into the in-place copy functionality? e.g.,

Why?  It's userspace's responsibility to get the above right.  If userspace fails
to provide a src_page when it doesn't want in-place copy, that's a userspace bug.