Subject: Re: [RFC v2] virtio: support packed ring

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 Apr 2018 05:37:19 +0300

On Tue, Apr 17, 2018 at 10:24:32AM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月17日 10:17, Michael S. Tsirkin wrote:
> > On Tue, Apr 17, 2018 at 10:11:58AM +0800, Jason Wang wrote:
> > > 
> > > On 2018年04月13日 15:15, Tiwei Bie wrote:
> > > > On Fri, Apr 13, 2018 at 12:30:24PM +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 <[email-masked]>
> > > > > > ---
> > > > > >     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(-)
> > > > [...]
> 
> [...]
> 
> > > > > It looks to me we should examine RING_EVENT_FLAGS_DESC in desc_event_flags
> > > > > instead of vq->event here. Spec does not forces to use evenf_off and
> > > > > event_wrap if event index is enabled.
> > > > > 
> > > > > > +		// FIXME: fix this!
> > > > > > +		needs_kick = ((off_wrap >> 15) == vq->wrap_counter) &&
> > > > > > +			     vring_need_event(off_wrap & ~(1<<15), new, old);
> > > > > Why need a & here?
> > > > Because wrap_counter (the most significant bit in off_wrap)
> > > > isn't part of the index.
> > > > 
> > > > > > +	} else {
> > > > > Need a smp_rmb() to make sure desc_event_flags was checked before flags.
> > > > I don't get your point, if my understanding is correct,
> > > > desc_event_flags is vq->vring_packed.device->flags. So
> > > > what's the "flags"?
> > > Sorry, I mean we need check device.flags before off_warp. So it needs an
> > > smp_rmb() in the middle.
> > It's best to just read all flags atomically as u32.
> 
> Yes it is.
> 
> > 
> > > It looks to me there's no guarantee that
> > > VRING_EVENT_F_DESC is set if event index is supported.
> > > 
> > > > > > +		needs_kick = (vq->vring_packed.device->flags !=
> > > > > > +			      cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
> > > > > > +	}
> > > > > > +	END_USE(vq);
> > > > > > +	return needs_kick;
> > > > > > +}
> > > > [...]
> > > > > > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > > > +			      void **ctx)
> > > > > > +{
> > > > > > +	struct vring_packed_desc *desc;
> > > > > > +	unsigned int i, j;
> > > > > > +
> > > > > > +	/* Clear data ptr. */
> > > > > > +	vq->desc_state[head].data = NULL;
> > > > > > +
> > > > > > +	i = head;
> > > > > > +
> > > > > > +	for (j = 0; j < vq->desc_state[head].num; j++) {
> > > > > > +		desc = &vq->vring_packed.desc[i];
> > > > > > +		vring_unmap_one_packed(vq, desc);
> > > > > > +		desc->flags = 0x0;
> > > > > Looks like this is unnecessary.
> > > > It's safer to zero it. If we don't zero it, after we
> > > > call virtqueue_detach_unused_buf_packed() which calls
> > > > this function, the desc is still available to the
> > > > device.
> > > Well detach_unused_buf_packed() should be called after device is stopped,
> > > otherwise even if you try to clear, there will still be a window that device
> > > may use it.
> > > 
> > > > > > +		i++;
> > > > > > +		if (i >= vq->vring_packed.num)
> > > > > > +			i = 0;
> > > > > > +	}
> > > > [...]
> > > > > > +static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> > > > > > +{
> > > > > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > > > +	u16 last_used_idx, wrap_counter, off_wrap;
> > > > > > +
> > > > > > +	START_USE(vq);
> > > > > > +
> > > > > > +	last_used_idx = vq->last_used_idx;
> > > > > > +	wrap_counter = vq->wrap_counter;
> > > > > > +
> > > > > > +	if (last_used_idx > vq->next_avail_idx)
> > > > > > +		wrap_counter ^= 1;
> > > > > > +
> > > > > > +	off_wrap = last_used_idx | (wrap_counter << 15);
> > > > > > +
> > > > > > +	/* We optimistically turn back on interrupts, then check if there was
> > > > > > +	 * more to do. */
> > > > > > +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> > > > > > +	 * either clear the flags bit or point the event index at the next
> > > > > > +	 * entry. Always do both to keep code simple. */
> > > > > > +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > > > > > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> > > > > > +						     VRING_EVENT_F_ENABLE;
> > > > > > +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> > > > > > +							vq->event_flags_shadow);
> > > > > > +	}
> > > > > A smp_wmb() is missed here?
> > > > > 
> > > > > > +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev, off_wrap);
> > > > > And according to the spec, it looks to me write a VRING_EVENT_F_ENABLE is
> > > > > sufficient here.
> > > > I didn't think much when implementing the event suppression
> > > > for packed ring previously. After I saw your comments, I found
> > > > something new. Indeed, unlike the split ring, for the packed
> > > > ring, spec doesn't say we must use VRING_EVENT_F_DESC when
> > > > EVENT_IDX is negotiated. So do you think below thought is
> > > > right or makes sense?
> > > > 
> > > > - For virtqueue_enable_cb_prepare(), we just need to enable
> > > >     the ring by setting flags to VRING_EVENT_F_ENABLE in any
> > > >     case.
> > > > 
> > > > - We will try to use VRING_EVENT_F_DESC (if EVENT_IDX is
> > > >     negotiated) only when we want to delay the interrupts
> > > >     virtqueue_enable_cb_delayed().
> > > This looks good to me.
> > I suspect this will lead to extra interrupts if host is fast.
> > So I think for now we should always use VRING_EVENT_F_DESC
> > if EVENT_IDX is negotiated.
> 
> Right, so if this is true, maybe we'd better force this in the spec?
> 
> Thanks

I did consider this.

Why it is the way it is:

- if we allow DISABLE it seems nicer to allow ENABLE as well
for symmetry.

- ENABLE is handy for hardware offload devices
where kicks do not trigger an exit so not worth bother
with, but interrupts still slow the system down so
event index might be worth it.

> > 
> > VRING_EVENT_F_DISABLE makes more sense to me.
> > 
> 
> [...]
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