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

Ryan Roberts
2024-10-14T12:14:42Z

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