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virtio: add support for indirect ring entries

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Support a new feature flag for indirect ring entries. These are ring
entries which point to a table of buffer descriptors.

The idea here is to increase the ring capacity by allowing a larger
effective ring size whereby the ring size dictates the number of
requests that may be outstanding, rather than the size of those
requests.

This should be most effective in the case of block I/O where we can
potentially benefit by concurrently dispatching a large number of
large requests. Even in the simple case of single segment block
requests, this results in a threefold increase in ring capacity.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This commit is contained in:
Mark McLoughlin 2009-06-17 11:38:28 +01:00 • committed by Anthony Liguori
parent 5774cf98ca
commit efeea6d048
3 changed files with 53 additions and 6 deletions
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4
hw/virtio.h
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@ -32,6 +32,8 @@
/* We notify when the ring is completely used, even if the guest is supressing
* callbacks */
#define VIRTIO_F_NOTIFY_ON_EMPTY 24
/* We support indirect buffer descriptors */
#define VIRTIO_RING_F_INDIRECT_DESC 28
/* A guest should never accept this. It implies negotiation is broken. */
#define VIRTIO_F_BAD_FEATURE 30
@ -41,6 +43,8 @@
#define VRING_DESC_F_NEXT 1
/* This marks a buffer as write-only (otherwise read-only). */
#define VRING_DESC_F_WRITE 2
/* This means the buffer contains a list of buffer descriptors. */
#define VRING_DESC_F_INDIRECT 4
/* This means don't notify other side when buffer added. */
#define VRING_USED_F_NO_NOTIFY 1