[RFC PATCH v1 35/57] virtio: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts]

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.

Updated multiple BUILD_BUG_ON() instances to test against page size
limits.

Wrap global variables that are initialized with PAGE_SIZE derived values
using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
deferred for boot-time page size builds.

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

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

 drivers/block/virtio_blk.c      |  2 +-
 drivers/virtio/virtio_balloon.c | 10 ++++++----
 net/9p/trans_virtio.c           |  4 ++--
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 194417abc1053..8a8960b609bc9 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -899,7 +899,7 @@ static ssize_t serial_show(struct device *dev,
 	int err;
 
 	/* sysfs gives us a PAGE_SIZE buffer */
-	BUILD_BUG_ON(PAGE_SIZE < VIRTIO_BLK_ID_BYTES);
+	BUILD_BUG_ON(PAGE_SIZE_MIN < VIRTIO_BLK_ID_BYTES);
 
 	buf[VIRTIO_BLK_ID_BYTES] = '\0';
 	err = virtblk_get_id(disk, buf);
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 54469277ca303..3818d894bd212 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -25,6 +25,7 @@
  * page units.
  */
 #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned int)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
+#define VIRTIO_BALLOON_PAGES_PER_PAGE_MAX (unsigned int)(PAGE_SIZE_MAX >> VIRTIO_BALLOON_PFN_SHIFT)
 #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
 /* Maximum number of (4k) pages to deflate on OOM notifications. */
 #define VIRTIO_BALLOON_OOM_NR_PAGES 256
@@ -138,7 +139,7 @@ static u32 page_to_balloon_pfn(struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);
 
-	BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
+	BUILD_BUG_ON(PAGE_SHIFT_MIN < VIRTIO_BALLOON_PFN_SHIFT);
 	/* Convert pfn from Linux page size to balloon page size. */
 	return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
 }
@@ -228,7 +229,7 @@ static void set_page_pfns(struct virtio_balloon *vb,
 {
 	unsigned int i;
 
-	BUILD_BUG_ON(VIRTIO_BALLOON_PAGES_PER_PAGE > VIRTIO_BALLOON_ARRAY_PFNS_MAX);
+	BUILD_BUG_ON(VIRTIO_BALLOON_PAGES_PER_PAGE_MAX > VIRTIO_BALLOON_ARRAY_PFNS_MAX);
 
 	/*
 	 * Set balloon pfns pointing at this page.
@@ -1042,8 +1043,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		 * host's base page size. However, it needs more work to report
 		 * that value. The hard-coded order would be fine currently.
 		 */
-#if defined(CONFIG_ARM64) && defined(CONFIG_ARM64_64K_PAGES)
-		vb->pr_dev_info.order = 5;
+#if defined(CONFIG_ARM64)
+		if (PAGE_SIZE == SZ_64K)
+			vb->pr_dev_info.order = 5;
 #endif
 
 		err = page_reporting_register(&vb->pr_dev_info);
diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 0b8086f58ad55..25b8253011cec 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -786,7 +786,7 @@ static struct virtio_driver p9_virtio_drv = {
 	.remove		= p9_virtio_remove,
 };
 
-static struct p9_trans_module p9_virtio_trans = {
+static DEFINE_GLOBAL_PAGE_SIZE_VAR(struct p9_trans_module, p9_virtio_trans, {
 	.name = "virtio",
 	.create = p9_virtio_create,
 	.close = p9_virtio_close,
@@ -804,7 +804,7 @@ static struct p9_trans_module p9_virtio_trans = {
 	.pooled_rbuffers = false,
 	.def = 1,
 	.owner = THIS_MODULE,
-};
+});
 
 /* The standard init function */
 static int __init p9_virtio_init(void)
-- 
2.43.0