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Subject[PATCH] virtio_balloon: drop virtio_balloon_stat_modern
Looks like we are better off sticking with the misaligned stat struct,
to reduce the amount of virtio 1 specific code in balloon. So let's do
it.

Add a detailed comment to reduce the chance people copy this bad example.

This also fixes a bug on BE architectures: tag should use
cpu_to_le16, not cpu_to_le32.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Just reposting so it's easier to apply.
Feel free to squash into previous patch if you think it's
neater.

include/uapi/linux/virtio_balloon.h | 33 +++++++++++++++++++++++++--------
drivers/virtio/virtio_balloon.c | 19 ++++---------------
2 files changed, 29 insertions(+), 23 deletions(-)

diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index f81b220..984169a 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -52,15 +52,32 @@ struct virtio_balloon_config {
#define VIRTIO_BALLOON_S_MEMTOT 5 /* Total amount of memory */
#define VIRTIO_BALLOON_S_NR 6

+/*
+ * Memory statistics structure.
+ * Driver fills an array of these structures and passes to device.
+ *
+ * NOTE: fields are laid out in a way that would make compiler add padding
+ * between and after fields, so we have to use compiler-specific attributes to
+ * pack it, to disable this padding. This also often causes compiler to
+ * generate suboptimal code.
+ *
+ * We maintain this statistics structure format for backwards compatibility,
+ * but don't follow this example.
+ *
+ * If implementing a similar structure, do something like the below instead:
+ * struct virtio_balloon_stat {
+ * __virtio16 tag;
+ * __u8 reserved[6];
+ * __virtio64 val;
+ * };
+ *
+ * In other words, add explicit reserved fields to align field and
+ * structure boundaries at field size, avoiding compiler padding
+ * without the packed attribute.
+ */
struct virtio_balloon_stat {
- __u16 tag;
- __u64 val;
+ __virtio16 tag;
+ __virtio64 val;
} __attribute__((packed));

-struct virtio_balloon_stat_modern {
- __le16 tag;
- __u8 reserved[6];
- __le64 val;
-};
-
#endif /* _LINUX_VIRTIO_BALLOON_H */
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 0583720..9db546e 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -77,10 +77,7 @@ struct virtio_balloon {

/* Memory statistics */
int need_stats_update;
- union {
- struct virtio_balloon_stat_modern stats[VIRTIO_BALLOON_S_NR];
- struct virtio_balloon_stat legacy_stats[VIRTIO_BALLOON_S_NR];
- };
+ struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];

/* To register callback in oom notifier call chain */
struct notifier_block nb;
@@ -93,10 +90,7 @@ static struct virtio_device_id id_table[] = {

static void stats_sg_init(struct virtio_balloon *vb, struct scatterlist *sg)
{
- if (virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
- sg_init_one(sg, vb->stats, sizeof(vb->stats));
- else
- sg_init_one(sg, vb->legacy_stats, sizeof(vb->legacy_stats));
+ sg_init_one(sg, vb->stats, sizeof(vb->stats));
}

static u32 page_to_balloon_pfn(struct page *page)
@@ -225,13 +219,8 @@ static inline void update_stat(struct virtio_balloon *vb, int idx,
u16 tag, u64 val)
{
BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
- if (virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1)) {
- vb->stats[idx].tag = cpu_to_le32(tag);
- vb->stats[idx].val = cpu_to_le64(val);
- } else {
- vb->legacy_stats[idx].tag = tag;
- vb->legacy_stats[idx].val = val;
- }
+ vb->stats[idx].tag = cpu_to_virtio16(vb->vdev, tag);
+ vb->stats[idx].val = cpu_to_virtio64(vb->vdev, val);
}

#define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)
--
MST

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