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Date: Wed, 15 Apr 2015 17:45:11 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Rusty Russell <rusty@...tcorp.com.au>
Cc: Cornelia Huck <cornelia.huck@...ibm.com>,
linux-kernel@...r.kernel.org, Pawel Moll <pawel.moll@....com>,
virtio-dev@...ts.oasis-open.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 0/6] virtio_balloon: virtio 1 support
On Wed, Apr 15, 2015 at 10:15:20AM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@...hat.com> writes:
> > On Tue, Apr 14, 2015 at 11:50:53AM +0200, Cornelia Huck wrote:
> >> On Tue, 14 Apr 2015 11:21:11 +0200
> >> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >>
> >> > 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.
> >>
> >> s/this/the existing statistics structure/
> >
> > existing seems redundant. What else? non-existing?
> >
> >> > + *
> >> > + * Do something like the below instead:
> >>
> >> If you want to implement a similar structure, do...
> >>
> >> Just that nobody gets the idea that they are supposed to implement new
> >> balloon statistics ;)
> >>
> >> > + * struct virtio_balloon_stat {
> >> > + * __virtio16 tag;
> >> > + * __u8 reserved[6];
> >> > + * __virtio64 val;
> >> > + * };
> >>
> >> (...)
> >>
> >> > @@ -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);
> >>
> >> Seems that nobody seemed to care much about statistics...
> >
> > Or about BE guests ;)
> >
> >> > + vb->stats[idx].val = cpu_to_virtio64(vb->vdev, val);
> >> > }
> >> >
> >> > #define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)
> >> >
> >>
> >> With these changes merged in:
> >>
> >> Acked-by: Cornelia Huck <cornelia.huck@...ibm.com>
> >
> >
> > OK, here's an updated incremental patch: only comment
> > changed.
>
> OK, I've merged this with one change:
>
> +static void stats_sg_init(struct virtio_balloon *vb, struct scatterlist *sg)
> +{
> + sg_init_one(sg, vb->stats, sizeof(vb->stats));
> +}
> +
> ...
> - sg_init_one(&sg, vb->stats, sizeof(vb->stats));
> + stats_sg_init(vb, &sg);
>
> This is no longer a meaningful change, so I removed it.
>
> Here's the final result:
>
> From: Michael S. Tsirkin <mst@...hat.com>
> Subject: virtio_balloon: transitional interface
>
> Virtio 1.0 doesn't include a modern balloon device.
> But it's not a big change to support a transitional
> balloon device: this has the advantage of supporting
> existing drivers, transparently.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> Acked-by: Cornelia Huck <cornelia.huck@...ibm.com>
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Fine by me.
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 6a356e344f82..9db546ebe5a1 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -214,8 +219,8 @@ static inline void update_stat(struct virtio_balloon *vb, int idx,
> u16 tag, u64 val)
> {
> BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
> - vb->stats[idx].tag = tag;
> - vb->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)
> @@ -283,18 +288,27 @@ static void virtballoon_changed(struct virtio_device *vdev)
>
> static inline s64 towards_target(struct virtio_balloon *vb)
> {
> - __le32 v;
> s64 target;
> + u32 num_pages;
>
> - virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages, &v);
> + virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages,
> + &num_pages);
>
> - target = le32_to_cpu(v);
> + /* Legacy balloon config space is LE, unlike all other devices. */
> + if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
> + num_pages = le32_to_cpu((__force __le32)num_pages);
> +
> + target = num_pages;
> return target - vb->num_pages;
> }
>
> static void update_balloon_size(struct virtio_balloon *vb)
> {
> - __le32 actual = cpu_to_le32(vb->num_pages);
> + u32 actual = vb->num_pages;
> +
> + /* Legacy balloon config space is LE, unlike all other devices. */
> + if (!virtio_has_feature(vb->vdev, VIRTIO_F_VERSION_1))
> + actual = (__force u32)cpu_to_le32(actual);
>
> virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
> &actual);
> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
> index 4b0488f20b2e..984169a819ee 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -25,6 +25,7 @@
> * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> * SUCH DAMAGE. */
> +#include <linux/types.h>
> #include <linux/virtio_ids.h>
> #include <linux/virtio_config.h>
>
> @@ -38,9 +39,9 @@
>
> struct virtio_balloon_config {
> /* Number of pages host wants Guest to give up. */
> - __le32 num_pages;
> + __u32 num_pages;
> /* Number of pages we've actually got in balloon. */
> - __le32 actual;
> + __u32 actual;
> };
>
> #define VIRTIO_BALLOON_S_SWAP_IN 0 /* Amount of memory swapped in */
> @@ -51,9 +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));
>
> #endif /* _LINUX_VIRTIO_BALLOON_H */
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