Messages in this thread | | | Subject | Re: [RFC v2] virtio: support packed ring | From | Jason Wang <> | Date | Tue, 24 Apr 2018 08:54:52 +0800 |
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On 2018年04月23日 17:29, Tiwei Bie wrote: > On Mon, Apr 23, 2018 at 01:42:14PM +0800, Jason Wang wrote: >> On 2018年04月01日 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@intel.com> >>> --- >>> 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
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