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

From: Ackerley Tng <redacted>

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 <seanjc@google.com>
Tested-by: Shivank Garg <redacted>
Signed-off-by: Ackerley Tng <redacted>
---
 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--;