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SubjectRe: [PATCH v3 0/6] virtio_balloon: virtio 1 support
On Tue, Apr 14, 2015 at 10:24:38AM +0200, Cornelia Huck wrote:
> On Tue, 14 Apr 2015 10:42:56 +0930
> Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> > "Michael S. Tsirkin" <mst@redhat.com> writes:
> > > On Wed, Apr 01, 2015 at 02:57:35PM +0200, Michael S. Tsirkin wrote:
> > >> Virtio 1.0 doesn't include a modern balloon device. At some point we'll likely
> > >> define an incompatible interface with a different ID and different
> > >> semantics. But for now, it's not a big effort to support a transitional
> > >> balloon device: this has the advantage of supporting existing drivers,
> > >> transparently, as well as transports that don't allow mixing virtio 0 and
> > >> virtio 1 devices. And balloon is an easy device to test, so it's also useful
> > >> for people to test virtio core handling of transitional devices.
> > >>
> > >> The only interface issue is with the stats buffer, which has misaligned
> > >> fields. We could leave it as is, but this sets a bad precedent that
> > >> others might copy by mistake.
> > >>
> > >> As we need to change stats code to do byteswaps for virtio 1.0, it seems easy
> > >> to fix by prepending a 6 byte reserved field. I also had to change config
> > >> structure field types from __le32 to __u32 to match other devices. This means
> > >> we need a couple of __force tags for legacy path but that seems minor.
> > >
> > > Rusty, what are your thoughts here?
> > > How about merging this for 4.1?
> >
> > I disagree with making any changes, other than allowing it to be used
> > with VIRTIO_F_VERSION_1.
> >
> > However it doesn't seem to bother anyone else, so I've applied it for
> > 4.1.
>
> I'm still not really convinced about the stats change either, FWIW.
> Still time to reconsider?
>
> And should we perhaps wait with merging until
> the spec change allowing version 1 has been accepted?

That's not how we did this historically: we require all parts
(spec,qemu,linux) to be available, but don't create specific order
between them. In particular, I'd strongly prefer not waiting until 4.2,
that would interfere with putting virtio 1 out to use in the field.

Since both Rusty and Cornelia are against virtio_balloon_stat_modern,
I accept this as the majority decision. And switching
over to __virtio tags found a bug, so I'm convinced now.

--->

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.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

----


diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index f81b220..164e0c2 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -52,15 +52,31 @@ 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 for backwards compatibility, but don't follow this example.
+ *
+ * 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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