[RFC PATCH v1 36/57] xen: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts <ryan.roberts@arm.com>]

To prepare for supporting boot-time page size selection, refactor code
to remove assumptions about PAGE_SIZE being compile-time constant. Code
intended to be equivalent when compile-time page size is active.

Allocate enough "frame_list" static storage in the balloon driver for
the maximum supported page size. Although continue to use only the first
PAGE_SIZE of the buffer at run-time to maintain existing behaviour.

Refactor xen_biovec_phys_mergeable() to convert ifdeffery to c if/else.
For compile-time page size, the compiler will choose one branch and
strip the dead one. For boot-time, it can be evaluated at run time.

Refactor a BUILD_BUG_ON to evaluate the limit (when the minimum
supported page size is selected at boot-time).

Reserve enough storage for max page size in "struct remap_data" and
"struct xenbus_map_node".

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

 drivers/xen/balloon.c              | 11 ++++++-----
 drivers/xen/biomerge.c             | 12 ++++++------
 drivers/xen/privcmd.c              |  2 +-
 drivers/xen/xenbus/xenbus_client.c |  5 +++--
 drivers/xen/xlate_mmu.c            |  6 +++---
 include/xen/page.h                 |  2 ++
 6 files changed, 21 insertions(+), 17 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 528395133b4f8..0ed5f6453af0e 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -131,7 +131,8 @@ struct balloon_stats balloon_stats;
 EXPORT_SYMBOL_GPL(balloon_stats);
 
 /* We increase/decrease in batches which fit in a page */
-static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];
+static xen_pfn_t frame_list[PAGE_SIZE_MAX / sizeof(xen_pfn_t)];
+#define FRAME_LIST_NR_ENTRIES (PAGE_SIZE / sizeof(xen_pfn_t))
 
 
 /* List of ballooned pages, threaded through the mem_map array. */
@@ -389,8 +390,8 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 	unsigned long i;
 	struct page   *page;
 
-	if (nr_pages > ARRAY_SIZE(frame_list))
-		nr_pages = ARRAY_SIZE(frame_list);
+	if (nr_pages > FRAME_LIST_NR_ENTRIES)
+		nr_pages = FRAME_LIST_NR_ENTRIES;
 
 	page = list_first_entry_or_null(&ballooned_pages, struct page, lru);
 	for (i = 0; i < nr_pages; i++) {
@@ -434,8 +435,8 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 	int ret;
 	LIST_HEAD(pages);
 
-	if (nr_pages > ARRAY_SIZE(frame_list))
-		nr_pages = ARRAY_SIZE(frame_list);
+	if (nr_pages > FRAME_LIST_NR_ENTRIES)
+		nr_pages = FRAME_LIST_NR_ENTRIES;
 
 	for (i = 0; i < nr_pages; i++) {
 		page = alloc_page(gfp);
diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
index 05a286d24f148..28f0887e40026 100644
--- a/drivers/xen/biomerge.c
+++ b/drivers/xen/biomerge.c
@@ -8,16 +8,16 @@
 bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 			       const struct page *page)
 {
-#if XEN_PAGE_SIZE == PAGE_SIZE
-	unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
-	unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
+	if (XEN_PAGE_SIZE == PAGE_SIZE) {
+		unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
+		unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
+
+		return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
+	}
 
-	return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
-#else
 	/*
 	 * XXX: Add support for merging bio_vec when using different page
 	 * size in Xen and Linux.
 	 */
 	return false;
-#endif
 }
diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 9563650dfbafc..847f7b806caf7 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -557,7 +557,7 @@ static long privcmd_ioctl_mmap_batch(
 	state.global_error  = 0;
 	state.version       = version;
 
-	BUILD_BUG_ON(((PAGE_SIZE / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE) != 0);
+	BUILD_BUG_ON(((PAGE_SIZE_MIN / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE_MAX) != 0);
 	/* mmap_batch_fn guarantees ret == 0 */
 	BUG_ON(traverse_pages_block(m.num, sizeof(xen_pfn_t),
 				    &pagelist, mmap_batch_fn, &state));
diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
index 51b3124b0d56c..99bde836c10c4 100644
--- a/drivers/xen/xenbus/xenbus_client.c
+++ b/drivers/xen/xenbus/xenbus_client.c
@@ -49,9 +49,10 @@
 
 #include "xenbus.h"
 
-#define XENBUS_PAGES(_grants)	(DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE))
+#define XENBUS_PAGES(_grants)		(DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE))
+#define XENBUS_PAGES_MAX(_grants)	(DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE_MIN))
 
-#define XENBUS_MAX_RING_PAGES	(XENBUS_PAGES(XENBUS_MAX_RING_GRANTS))
+#define XENBUS_MAX_RING_PAGES		(XENBUS_PAGES_MAX(XENBUS_MAX_RING_GRANTS))
 
 struct xenbus_map_node {
 	struct list_head next;
diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
index f17c4c03db30c..a757c801a7542 100644
--- a/drivers/xen/xlate_mmu.c
+++ b/drivers/xen/xlate_mmu.c
@@ -74,9 +74,9 @@ struct remap_data {
 	int mapped;
 
 	/* Hypercall parameters */
-	int h_errs[XEN_PFN_PER_PAGE];
-	xen_ulong_t h_idxs[XEN_PFN_PER_PAGE];
-	xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE];
+	int h_errs[XEN_PFN_PER_PAGE_MAX];
+	xen_ulong_t h_idxs[XEN_PFN_PER_PAGE_MAX];
+	xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE_MAX];
 
 	int h_iter;	/* Iterator */
 };
diff --git a/include/xen/page.h b/include/xen/page.h
index 285677b42943a..86683a30038a3 100644
--- a/include/xen/page.h
+++ b/include/xen/page.h
@@ -21,6 +21,8 @@
 	((page_to_pfn(page)) << (PAGE_SHIFT - XEN_PAGE_SHIFT))
 
 #define XEN_PFN_PER_PAGE	(PAGE_SIZE / XEN_PAGE_SIZE)
+#define XEN_PFN_PER_PAGE_MIN	(PAGE_SIZE_MIN / XEN_PAGE_SIZE)
+#define XEN_PFN_PER_PAGE_MAX	(PAGE_SIZE_MAX / XEN_PAGE_SIZE)
 
 #define XEN_PFN_DOWN(x)	((x) >> XEN_PAGE_SHIFT)
 #define XEN_PFN_UP(x)	(((x) + XEN_PAGE_SIZE-1) >> XEN_PAGE_SHIFT)
-- 
2.43.0