iommu/amd: Introduce first_pte_l7() helper

Given an arbitrary pte that is part of a large mapping, this function
returns the first pte of the series (and optionally the mapped size and
number of PTEs)
It will be re-used in a subsequent patch to replace an existing L7
mapping.

Fixes: 6d568ef9 ("iommu/amd: Allow downgrading page-sizes in alloc_pte()")
Signed-off-by: Andrei Dulea <adulea@amazon.de>

drivers/iommu/amd_iommu.c

@@ -501,6 +501,29 @@ static void iommu_uninit_device(struct device *dev)
 	 */
 }
 
+/*
+ * Helper function to get the first pte of a large mapping
+ */
+static u64 *first_pte_l7(u64 *pte, unsigned long *page_size,
+			 unsigned long *count)
+{
+	unsigned long pte_mask, pg_size, cnt;
+	u64 *fpte;
+
+	pg_size  = PTE_PAGE_SIZE(*pte);
+	cnt      = PAGE_SIZE_PTE_COUNT(pg_size);
+	pte_mask = ~((cnt << 3) - 1);
+	fpte     = (u64 *)(((unsigned long)pte) & pte_mask);
+
+	if (page_size)
+		*page_size = pg_size;
+
+	if (count)
+		*count = cnt;
+
+	return fpte;
+}
+
 /****************************************************************************
  *
  * Interrupt handling functions
@@ -1567,17 +1590,12 @@ static u64 *fetch_pte(struct protection_domain *domain,
 		*page_size = PTE_LEVEL_PAGE_SIZE(level);
 	}
 
-	if (PM_PTE_LEVEL(*pte) == 0x07) {
-		unsigned long pte_mask;
-
-		/*
-		 * If we have a series of large PTEs, make
-		 * sure to return a pointer to the first one.
-		 */
-		*page_size = pte_mask = PTE_PAGE_SIZE(*pte);
-		pte_mask   = ~((PAGE_SIZE_PTE_COUNT(pte_mask) << 3) - 1);
-		pte        = (u64 *)(((unsigned long)pte) & pte_mask);
-	}
+	/*
+	 * If we have a series of large PTEs, make
+	 * sure to return a pointer to the first one.
+	 */
+	if (PM_PTE_LEVEL(*pte) == PAGE_MODE_7_LEVEL)
+		pte = first_pte_l7(pte, page_size, NULL);
 
 	return pte;
 }