[RFC PATCH v2 04/23] KVM: TDX: Introduce tdx_clear_folio() to clear huge pages

[Yan Zhao <yan.y.zhao@intel.com>]

After removing or reclaiming a guest private page or a control page from a
TD, zero the physical page using movdir64b(), enabling the kernel to reuse
the pages.

Introduce the function tdx_clear_folio() to zero out physical memory using
movdir64b(), starting from the page at "start_idx" within a "folio" and
spanning "npages" contiguous PFNs.

Convert tdx_clear_page() to be a helper function to facilitate the
zeroing of 4KB pages.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
RFC v2:
- Add tdx_clear_folio().
- Drop inner loop _tdx_clear_page() and move __mb() outside of the loop.
  (Rick)
- Use C99-style definition of variables inside a for loop.
- Note: [1] also changes tdx_clear_page(). RFC v2 is not based on [1] now.

[1] https://lore.kernel.org/all/20250724130354.79392-2-adrian.hunter@intel.com

RFC v1:
- split out, let tdx_clear_page() accept level.
---
 arch/x86/kvm/vmx/tdx.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 8eaf8431c5f1..4fabefb27135 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -277,18 +277,21 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
 	vcpu->cpu = -1;
 }
 
-static void tdx_clear_page(struct page *page)
+static void tdx_clear_folio(struct folio *folio, unsigned long start_idx,
+			    unsigned long npages)
 {
 	const void *zero_page = (const void *) page_to_virt(ZERO_PAGE(0));
-	void *dest = page_to_virt(page);
-	unsigned long i;
 
 	/*
 	 * The page could have been poisoned.  MOVDIR64B also clears
 	 * the poison bit so the kernel can safely use the page again.
 	 */
-	for (i = 0; i < PAGE_SIZE; i += 64)
-		movdir64b(dest + i, zero_page);
+	for (unsigned long j = 0; j < npages; j++) {
+		void *dest = page_to_virt(folio_page(folio, start_idx + j));
+
+		for (unsigned long i = 0; i < PAGE_SIZE; i += 64)
+			movdir64b(dest + i, zero_page);
+	}
 	/*
 	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
 	 * from seeing potentially poisoned cache.
@@ -296,6 +299,13 @@ static void tdx_clear_page(struct page *page)
 	__mb();
 }
 
+static inline void tdx_clear_page(struct page *page)
+{
+	struct folio *folio = page_folio(page);
+
+	tdx_clear_folio(folio, folio_page_idx(folio, page), 1);
+}
+
 static void tdx_no_vcpus_enter_start(struct kvm *kvm)
 {
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
@@ -1736,7 +1746,7 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
 		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
 		return -EIO;
 	}
-	tdx_clear_page(page);
+	tdx_clear_folio(folio, folio_page_idx(folio, page), KVM_PAGES_PER_HPAGE(level));
 	tdx_pamt_put(page, level);
 	tdx_unpin(kvm, page);
 	return 0;
-- 
2.43.2