Re: [RFC][Patch v8 5/7] virtio: Enables to add a single descriptor to the host

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 2/6/19 1:03 PM, Michael S. Tsirkin wrote:
> On Wed, Feb 06, 2019 at 07:56:37AM -0500, Nitesh Narayan Lal wrote:
>> On 2/5/19 3:49 PM, Michael S. Tsirkin wrote:
>>> On Mon, Feb 04, 2019 at 03:18:52PM -0500, Nitesh Narayan Lal wrote:
>>>> This patch enables the caller to expose a single buffers to the
>>>> other end using vring descriptor. It also allows the caller to
>>>> perform this action in synchornous manner by using virtqueue_kick_sync.
>>>>
>>>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>>> I am not sure why do we need this API. Polling in guest
>>> until host runs isn't great either since these
>>> might be running on the same host CPU.
>> True.
>>
>> However, my understanding is that the existing API such as
>> virtqueue_add_outbuf() requires an allocation which will be problematic
>> for my implementation.
> Not with a single s/g entry, no.
Didn't know this. I will re-check.
>
>> Although I am not blocking the allocation path during normal Linux
>> kernel usage as even if one of the zone is locked the other zone could
>> be used to get free pages.
>
> I am a bit confused about locking, 
My bad, I think I created the confusion.
> I was under the impression
> that you are not calling virtio under a zone lock.

Yeap. Your understanding is correct.
I will re-visit this and correct it in the next version.
> FYI doing that was nacked by Linus.
>
>
>> But during the initial boot time (device initialization), in certain
>> situations the allocation can only come from a single zone, acquiring a
>> lock on it may result in a deadlock situation.
>>
>>>
>>>
>>>> ---
>>>>  drivers/virtio/virtio_ring.c | 72 ++++++++++++++++++++++++++++++++++++
>>>>  include/linux/virtio.h       |  4 ++
>>>>  2 files changed, 76 insertions(+)
>>>>
>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>>> index cd7e755484e3..93c161ac6a28 100644
>>>> --- a/drivers/virtio/virtio_ring.c
>>>> +++ b/drivers/virtio/virtio_ring.c
>>>> @@ -1695,6 +1695,52 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>>>>  					out_sgs, in_sgs, data, ctx, gfp);
>>>>  }
>>>>  
>>>> +/**
>>>> + * virtqueue_add_desc - add a buffer to a chain using a vring desc
>>>> + * @vq: the struct virtqueue we're talking about.
>>>> + * @addr: address of the buffer to add.
>>>> + * @len: length of the buffer.
>>>> + * @in: set if the buffer is for the device to write.
>>>> + *
>>>> + * Returns zero or a negative error (ie. ENOSPC, ENOMEM, EIO).
>>>> + */
>>>> +int virtqueue_add_desc(struct virtqueue *_vq, u64 addr, u32 len, int in)
>>>> +{
>>>> +	struct vring_virtqueue *vq = to_vvq(_vq);
>>>> +	struct vring_desc *desc = vq->split.vring.desc;
>>>> +	u16 flags = in ? VRING_DESC_F_WRITE : 0;
>>>> +	unsigned int i;
>>>> +	void *data = (void *)addr;
>>>> +	int avail_idx;
>>>> +
>>>> +	/* Sanity check */
>>>> +	if (!_vq)
>>>> +		return -EINVAL;
>>>> +
>>>> +	START_USE(vq);
>>>> +	if (unlikely(vq->broken)) {
>>>> +		END_USE(vq);
>>>> +		return -EIO;
>>>> +	}
>>>> +
>>>> +	i = vq->free_head;
>>>> +	flags &= ~VRING_DESC_F_NEXT;
>>>> +	desc[i].flags = cpu_to_virtio16(_vq->vdev, flags);
>>>> +	desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
>>>> +	desc[i].len = cpu_to_virtio32(_vq->vdev, len);
>>>> +
>>>> +	vq->vq.num_free--;
>>>> +	vq->free_head = virtio16_to_cpu(_vq->vdev, desc[i].next);
>>>> +	vq->split.desc_state[i].data = data;
>>>> +	vq->split.avail_idx_shadow = 1;
>>>> +	avail_idx = vq->split.avail_idx_shadow;
>>>> +	vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, avail_idx);
>>>> +	vq->num_added = 1;
>>>> +	END_USE(vq);
>>>> +	return 0;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(virtqueue_add_desc);
>>>> +
>>>>  /**
>>>>   * virtqueue_add_sgs - expose buffers to other end
>>>>   * @vq: the struct virtqueue we're talking about.
>>>> @@ -1842,6 +1888,32 @@ bool virtqueue_notify(struct virtqueue *_vq)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(virtqueue_notify);
>>>>  
>>>> +/**
>>>> + * virtqueue_kick_sync - update after add_buf and busy wait till update is done
>>>> + * @vq: the struct virtqueue
>>>> + *
>>>> + * After one or more virtqueue_add_* calls, invoke this to kick
>>>> + * the other side. Busy wait till the other side is done with the update.
>>>> + *
>>>> + * Caller must ensure we don't call this with other virtqueue
>>>> + * operations at the same time (except where noted).
>>>> + *
>>>> + * Returns false if kick failed, otherwise true.
>>>> + */
>>>> +bool virtqueue_kick_sync(struct virtqueue *vq)
>>>> +{
>>>> +	u32 len;
>>>> +
>>>> +	if (likely(virtqueue_kick(vq))) {
>>>> +		while (!virtqueue_get_buf(vq, &len) &&
>>>> +		       !virtqueue_is_broken(vq))
>>>> +			cpu_relax();
>>>> +		return true;
>>>> +	}
>>>> +	return false;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(virtqueue_kick_sync);
>>>> +
>>>>  /**
>>>>   * virtqueue_kick - update after add_buf
>>>>   * @vq: the struct virtqueue
>>>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>>>> index fa1b5da2804e..58943a3a0e8d 100644
>>>> --- a/include/linux/virtio.h
>>>> +++ b/include/linux/virtio.h
>>>> @@ -57,6 +57,10 @@ int virtqueue_add_sgs(struct virtqueue *vq,
>>>>  		      unsigned int in_sgs,
>>>>  		      void *data,
>>>>  		      gfp_t gfp);
>>>> +/* A desc with this init id is treated as an invalid desc */
>>>> +int virtqueue_add_desc(struct virtqueue *_vq, u64 addr, u32 len, int in);
>>>> +
>>>> +bool virtqueue_kick_sync(struct virtqueue *vq);
>>>>  
>>>>  bool virtqueue_kick(struct virtqueue *vq);
>>>>  
>>>> -- 
>>>> 2.17.2
>> -- 
>> Regards
>> Nitesh
>>
>
>
-- 
Regards
Nitesh


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