Re: [PATCH v6 03/11] x86/virt/tdx: Add tdx_alloc/free_control_page() helpers

[Chao Gao]

On Mon, May 25, 2026 at 07:35:07PM -0700, Rick Edgecombe wrote:
>From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
>Add helpers to use when allocating or preparing pages that are handed to
>the TDX-Module for use as control/S-EPT pages, and thus need Dynamic PAMT

For the whole series: s/TDX-Module/TDX module, to match the existing
convention.

>+/*
>+ * Calculate the arg needed for operating on the DPAMT backing for
>+ * a given 4KB page.
>+ */
>+static u64 pamt_2mb_arg(kvm_pfn_t pfn)
>+{
>+	unsigned long hpa_2mb = ALIGN_DOWN(pfn << PAGE_SHIFT, PMD_SIZE);

The changelog and the comment don't explain why the pfn is aligned down to a
2MB boundary. And ...

>+
>+	return hpa_2mb | TDX_PS_2M;
>+}
>+
>+/* Add PAMT backing for the given page. */

... this adds PAMT backing for the whole 2MB region containing the given page,
not just the given page itself. Is it worth clarifying in the comment?

<snip>

>+/*
>+ * Return a page that can be gifted to the TDX-Module for use as a "control"
>+ * page, i.e. pages that are used for control and S-EPT structures for a given
>+ * TDX guest, and bound to said guest's HKID and thus obtain TDX protections,
>+ * including PAMT tracking.
>+ */

IIUC, S-EPT structures are not allocated through this helper. Why mention it?

>+struct page *tdx_alloc_control_page(void)
>+{
>+	struct page *page;
>+
>+	page = alloc_page(GFP_KERNEL_ACCOUNT);
>+	if (!page)
>+		return NULL;
>+
>+	if (tdx_pamt_get(page_to_pfn(page))) {
>+		__free_page(page);
>+		return NULL;
>+	}
>+
>+	return page;
>+}
>+EXPORT_SYMBOL_FOR_KVM(tdx_alloc_control_page);
>+
>+/*
>+ * Free a page that was gifted to the TDX-Module for use as a control/S-EPT
>+ * page. After this, the page is no longer protected by TDX.
>+ */

Ditto.

>+void tdx_free_control_page(struct page *page)
>+{
>+	if (!page)
>+		return;
>+
>+	tdx_pamt_put(page_to_pfn(page));
>+	__free_page(page);
>+}
>+EXPORT_SYMBOL_FOR_KVM(tdx_free_control_page);

With above nits fixed,

Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>