On Thu, Mar 10, 2011 at 12:19:42PM +1030, Rusty Russell wrote:
> Here's an old patch where I played with implementing this:
...
>
> virtio: put last_used and last_avail index into ring itself.
>
> Generally, the other end of the virtio ring doesn't need to see where
> you're up to in consuming the ring. However, to completely understand
> what's going on from the outside, this information must be exposed.
> For example, if you want to save and restore a virtio_ring, but you're
> not the consumer because the kernel is using it directly.
>
> Fortunately, we have room to expand:
This seems to be true for x86 kvm and lguest but is it true
for s390?
err = vmem_add_mapping(config->address,
vring_size(config->num,
KVM_S390_VIRTIO_RING_ALIGN));
if (err)
goto out;
vq = vring_new_virtqueue(config->num, KVM_S390_VIRTIO_RING_ALIGN,
vdev, (void *) config->address,
kvm_notify, callback, name);
if (!vq) {
err = -ENOMEM;
goto unmap;
}
> the ring is always a whole number
> of pages and there's hundreds of bytes of padding after the avail ring
> and the used ring, whatever the number of descriptors (which must be a
> power of 2).
>
> We add a feature bit so the guest can tell the host that it's writing
> out the current value there, if it wants to use that.
>
> Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> ---
....
> --- a/include/linux/virtio_ring.h
> +++ b/include/linux/virtio_ring.h
> @@ -29,6 +29,9 @@
> /* We support indirect buffer descriptors */
> #define VIRTIO_RING_F_INDIRECT_DESC 28
>
> +/* We publish our last-seen used index at the end of the avail ring. */
> +#define VIRTIO_RING_F_PUBLISH_INDICES 29
> +
> /* Virtio ring descriptors: 16 bytes. These can chain together via "next". */
> struct vring_desc
> {
> @@ -87,6 +90,7 @@ struct vring {
> * __u16 avail_flags;
> * __u16 avail_idx;
> * __u16 available[num];
> + * __u16 last_used_idx;
> *
> * // Padding to the next align boundary.
> * char pad[];
> @@ -95,6 +99,7 @@ struct vring {
> * __u16 used_flags;
> * __u16 used_idx;
> * struct vring_used_elem used[num];
> + * __u16 last_avail_idx;
> * };
> */
> static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> @@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign
> {
> return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
> + align - 1) & ~(align - 1))
> - + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
> + + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2;
> }
>
> +/* We publish the last-seen used index at the end of the available ring, and
> + * vice-versa. These are at the end for backwards compatibility. */
> +#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num])
> +#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
> +
Will this last bit work on s390?
If I understand correctly the memory is allocated by host there?
> #ifdef __KERNEL__
> #include <linux/irqreturn.h>
> struct virtio_device;
--
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