Michael S. Tsirkin
2015-Apr-14 10:01 UTC
[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 at de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst at 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
Cornelia Huck
2015-Apr-14 10:13 UTC
[PATCH] virtio_balloon: drop virtio_balloon_stat_modern
On Tue, 14 Apr 2015 12:01:13 +0200 "Michael S. Tsirkin" <mst at redhat.com> wrote:> 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 at de.ibm.com> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com> > --- > > Just reposting so it's easier to apply. > Feel free to squash into previous patch if you think it's > neater.+1 for squashing from me. My A-b would also apply to the squashed patch.> > include/uapi/linux/virtio_balloon.h | 33 +++++++++++++++++++++++++-------- > drivers/virtio/virtio_balloon.c | 19 ++++--------------- > 2 files changed, 29 insertions(+), 23 deletions(-)
Rusty Russell
2015-Apr-15 03:15 UTC
[PATCH] virtio_balloon: drop virtio_balloon_stat_modern
Cornelia Huck <cornelia.huck at de.ibm.com> writes:> On Tue, 14 Apr 2015 12:01:13 +0200 > "Michael S. Tsirkin" <mst at redhat.com> wrote: > >> 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 at de.ibm.com> >> Signed-off-by: Michael S. Tsirkin <mst at redhat.com> >> --- >> >> Just reposting so it's easier to apply. >> Feel free to squash into previous patch if you think it's >> neater. > > +1 for squashing from me. My A-b would also apply to the squashed patch.Yes, I already squashed. TBH, I would have squashed the next patches into one too, that simply got rid of the virtio_device_is_legacy_only() function and all callers. Thanks, Rusty.