Re: [RFC v2] virtio: support packed ring
From: Jason Wang
Date: Mon Apr 23 2018 - 20:55:05 EST
On 2018å04æ23æ 17:29, Tiwei Bie wrote:
On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang wrote:
On 2018å04æ01æ 22:12, Tiwei Bie wrote:
Hello everyone,
This RFC implements packed ring support for virtio driver.
The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
Minor changes are needed for the vhost code, e.g. to kick the guest.
TODO:
- Refinements and bug fixes;
- Split into small patches;
- Test indirect descriptor support;
- Test/fix event suppression support;
- Test devices other than net;
RFC v1 -> RFC v2:
- Add indirect descriptor support - compile test only;
- Add event suppression supprt - compile test only;
- Move vring_packed_init() out of uapi (Jason, MST);
- Merge two loops into one in virtqueue_add_packed() (Jason);
- Split vring_unmap_one() for packed ring and split ring (Jason);
- Avoid using '%' operator (Jason);
- Rename free_head -> next_avail_idx (Jason);
- Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
- Some other refinements and bug fixes;
Thanks!
Signed-off-by: Tiwei Bie <tiwei.bie@xxxxxxxxx>
---
drivers/virtio/virtio_ring.c | 1094 +++++++++++++++++++++++++++++-------
include/linux/virtio_ring.h | 8 +-
include/uapi/linux/virtio_config.h | 12 +-
include/uapi/linux/virtio_ring.h | 61 ++
4 files changed, 980 insertions(+), 195 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 71458f493cf8..0515dca34d77 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -58,14 +58,15 @@
[...]
+
+ if (vq->indirect) {
+ u32 len;
+
+ desc = vq->desc_state[head].indir_desc;
+ /* Free the indirect table, if any, now that it's unmapped. */
+ if (!desc)
+ goto out;
+
+ len = virtio32_to_cpu(vq->vq.vdev,
+ vq->vring_packed.desc[head].len);
+
+ BUG_ON(!(vq->vring_packed.desc[head].flags &
+ cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we
can safely remove this BUG_ON() here.
+ BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
Len could be ignored for used descriptor according to the spec, so we need
remove this BUG_ON() too.
Yeah, you're right! The BUG_ON() isn't right. I'll remove it.
And I think something related to this in the spec isn't very
clear currently.
In the spec, there are below words:
https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272
"""
In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE
is reserved and is ignored by the device.
"""
So when device writes back an used descriptor in this case,
device may not set the VIRTQ_DESC_F_WRITE flag as the flag
is reserved and should be ignored.
https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170
"""
Element Length is reserved for used descriptors without the
VIRTQ_DESC_F_WRITE flag, and is ignored by drivers.
"""
And this is the way how driver ignores the `len` in an used
descriptor.
https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241
"""
To increase ring capacity the driver can store a (read-only
by the device) table of indirect descriptors anywhere in memory,
and insert a descriptor in the main virtqueue (with \field{Flags}
bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element
containing this indirect descriptor table;
"""
So the indirect descriptors in the table are read-only by
the device. And the only descriptor which is writeable by
the device is the descriptor in the main virtqueue (with
Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the
`len` in this descriptor, we won't be able to get the
length of the data written by the device.
So I think the `len` in this descriptor will carry the
length of the data written by the device (if the buffers
are writable to the device) even if the VIRTQ_DESC_F_WRITE
isn't set by the device. How do you think?
Yes I think so. But we'd better need clarification from Michael.
The reason is we don't touch descriptor ring in the case of split, so
BUG_ON()s may help there.
+
+ for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
+ vring_unmap_one_packed(vq, &desc[j]);
+
+ kfree(desc);
+ vq->desc_state[head].indir_desc = NULL;
+ } else if (ctx) {
+ *ctx = vq->desc_state[head].indir_desc;
+ }
+
+out:
+ return vq->desc_state[head].num;
+}
+
+static inline bool more_used_split(const struct vring_virtqueue *vq)
{
return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev, vq->vring.used->idx);
}
+static inline bool more_used_packed(const struct vring_virtqueue *vq)
+{
+ u16 last_used, flags;
+ bool avail, used;
+
+ if (vq->vq.num_free == vq->vring_packed.num)
+ return false;
+
+ last_used = vq->last_used_idx;
+ flags = virtio16_to_cpu(vq->vq.vdev,
+ vq->vring_packed.desc[last_used].flags);
+ avail = flags & VRING_DESC_F_AVAIL(1);
+ used = flags & VRING_DESC_F_USED(1);
+
+ return avail == used;
+}
This looks interesting, spec said:
"
Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an
available descriptor and
equal for a used descriptor.
Note that this observation is mostly useful for sanity-checking as these are
necessary but not sufficient
conditions - for example, all descriptors are zero-initialized. To detect
used and available descriptors it is
possible for drivers and devices to keep track of the last observed value of
VIRTQ_DESC_F_USED/VIRTQ_-
DESC_F_AVAIL. Other techniques to detect
VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes
might also be possible.
"
So it looks to me it was not sufficient, looking at the example codes in
spec, do we need to track last seen used_wrap_counter here?
I don't think we have to track used_wrap_counter in
driver. There was a discussion on this:
https://lists.oasis-open.org/archives/virtio-dev/201802/msg00177.html
And after that, below sentence was added (it's also
in the above words you quoted):
"""
Other techniques to detect
VIRTQ_DESC_F_AVAIL/VIRTQ_DESC_F_USED bit changes
might also be possible.
"""
Best regards,
Tiwei Bie
I see, the extra condition "if (vq->vq.num_free ==
vq->vring_packed.num)" help in this case.
Thanks
Thanks