x86/tdx: Eliminate duplicate code in tdx_clear_page()

tdx_clear_page() and reset_tdx_pages() duplicate the TDX page clearing
logic.  Rename reset_tdx_pages() to tdx_quirk_reset_paddr() and create
tdx_quirk_reset_page() to call tdx_quirk_reset_paddr() and be used in
place of tdx_clear_page().

The new name reflects that, in fact, the clearing is necessary only for
hardware with a certain quirk.  That is dealt with in a subsequent patch
but doing the rename here avoids additional churn.

Note reset_tdx_pages() is slightly different from tdx_clear_page() because,
more appropriately, it uses mb() in place of __mb().  Except when extra
debugging is enabled (kcsan at present), mb() just calls __mb().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kas@kernel.org>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Acked-by: Vishal Annapurve <vannapurve@google.com>
Link: https://lore.kernel.org/all/20250819155811.136099-2-adrian.hunter%40intel.com

arch/x86/include/asm/tdx.h

@@ -131,6 +131,8 @@ int tdx_guest_keyid_alloc(void);
 u32 tdx_get_nr_guest_keyids(void);
 void tdx_guest_keyid_free(unsigned int keyid);
 
+void tdx_quirk_reset_page(struct page *page);
+
 struct tdx_td {
 	/* TD root structure: */
 	struct page *tdr_page;

arch/x86/kvm/vmx/tdx.c

@@ -281,25 +281,6 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
 	vcpu->cpu = -1;
 }
 
-static void tdx_clear_page(struct page *page)
-{
-	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);
-	/*
-	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
-	 * from seeing potentially poisoned cache.
-	 */
-	__mb();
-}
-
 static void tdx_no_vcpus_enter_start(struct kvm *kvm)
 {
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
@@ -345,7 +326,7 @@ static int tdx_reclaim_page(struct page *page)
 
 	r = __tdx_reclaim_page(page);
 	if (!r)
-		tdx_clear_page(page);
+		tdx_quirk_reset_page(page);
 	return r;
 }
 
@@ -593,7 +574,7 @@ static void tdx_reclaim_td_control_pages(struct kvm *kvm)
 		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err);
 		return;
 	}
-	tdx_clear_page(kvm_tdx->td.tdr_page);
+	tdx_quirk_reset_page(kvm_tdx->td.tdr_page);
 
 	__free_page(kvm_tdx->td.tdr_page);
 	kvm_tdx->td.tdr_page = NULL;
@@ -1714,7 +1695,7 @@ 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_quirk_reset_page(page);
 	tdx_unpin(kvm, page);
 	return 0;
 }

arch/x86/virt/vmx/tdx/tdx.c

@@ -637,7 +637,7 @@ static int tdmrs_set_up_pamt_all(struct tdmr_info_list *tdmr_list,
  * clear these pages.  Note this function doesn't flush cache of
  * these TDX private pages.  The caller should make sure of that.
  */
-static void reset_tdx_pages(unsigned long base, unsigned long size)
+static void tdx_quirk_reset_paddr(unsigned long base, unsigned long size)
 {
 	const void *zero_page = (const void *)page_address(ZERO_PAGE(0));
 	unsigned long phys, end;
@@ -654,9 +654,15 @@ static void reset_tdx_pages(unsigned long base, unsigned long size)
 	mb();
 }
 
+void tdx_quirk_reset_page(struct page *page)
+{
+	tdx_quirk_reset_paddr(page_to_phys(page), PAGE_SIZE);
+}
+EXPORT_SYMBOL_GPL(tdx_quirk_reset_page);
+
 static void tdmr_reset_pamt(struct tdmr_info *tdmr)
 {
-	tdmr_do_pamt_func(tdmr, reset_tdx_pages);
+	tdmr_do_pamt_func(tdmr, tdx_quirk_reset_paddr);
 }
 
 static void tdmrs_reset_pamt_all(struct tdmr_info_list *tdmr_list)