On Fri, Jul 31, 2026, Xiaoyao Li wrote: > On 7/29/2026 8:35 AM, 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. > > > > 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 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?
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/[email protected]
