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

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Date:   Wed, 6 Feb 2019 07:56:37 -0500
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, lcapitulino@...hat.com, pagupta@...hat.com,
        wei.w.wang@...el.com, yang.zhang.wz@...il.com, riel@...riel.com,
        david@...hat.com, dodgen@...gle.com, konrad.wilk@...cle.com,
        dhildenb@...hat.com, aarcange@...hat.com
Subject: Re: [RFC][Patch v8 5/7] virtio: Enables to add a single descriptor to
 the host


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@...hat.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.
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.
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



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