linux-kernel - Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F

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Date:   Wed, 12 Jul 2017 21:29:00 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, david@...hat.com, cornelia.huck@...ibm.com,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, virtio-dev@...ts.oasis-open.org,
        yang.zhang.wz@...il.com, quan.xu@...yun.com
Subject: Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

On 07/12/2017 09:06 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 12, 2017 at 08:40:18PM +0800, Wei Wang wrote:
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index 28b0e96..9f27101 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -57,8 +57,28 @@ int virtqueue_add_sgs(struct virtqueue *vq,
>>   		      void *data,
>>   		      gfp_t gfp);
>>   
>> +/* A desc with this init id is treated as an invalid desc */
>> +#define VIRTQUEUE_DESC_ID_INIT UINT_MAX
>> +int virtqueue_add_chain_desc(struct virtqueue *_vq,
>> +			     uint64_t addr,
>> +			     uint32_t len,
>> +			     unsigned int *head_id,
>> +			     unsigned int *prev_id,
>> +			     bool in);
>> +
>> +int virtqueue_add_chain(struct virtqueue *_vq,
>> +			unsigned int head,
>> +			bool indirect,
>> +			struct vring_desc *indirect_desc,
>> +			void *data,
>> +			void *ctx);
>> +
>>   bool virtqueue_kick(struct virtqueue *vq);
>>   
>> +bool virtqueue_kick_sync(struct virtqueue *vq);
>> +
>> +bool virtqueue_kick_async(struct virtqueue *vq, wait_queue_head_t wq);
>> +
>>   bool virtqueue_kick_prepare(struct virtqueue *vq);
>>   
>>   bool virtqueue_notify(struct virtqueue *vq);
> I don't much care for this API. It does exactly what balloon needs,
> but at cost of e.g. transparently busy-waiting. Unlikely to be
> a good fit for anything else.

If you were referring to this API - virtqueue_add_chain_desc():

Busy waiting only happens when the vq is full (i.e. no desc left). If
necessary, I think we can add an input parameter like
"bool busywaiting", then the caller can decide to simply get a -ENOSPC
or busy wait to add when no desc is available.

>
> If you don't like my original _first/_next/_last, you will
> need to come up with something else.

I thought the above virtqueue_add_chain_des() performs the same
functionality as _first/next/last, which are used to grab descs from the
vq and chain them together. If not, could you please elaborate the
usage of the original proposal?

Best,
Wei