From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wei Wang <wei.w.wang@intel.com>
Subject: Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG
Date: Wed, 12 Jul 2017 21:29:00 +0800
Message-ID: <5966241C.9060503@intel.com>
References: <1499863221-16206-1-git-send-email-wei.w.wang@intel.com> <1499863221-16206-6-git-send-email-wei.w.wang@intel.com> <20170712160129-mutt-send-email-mst@kernel.org>
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Cc: linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
 virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
 linux-mm@kvack.org, david@redhat.com, cornelia.huck@de.ibm.com,
 akpm@linux-foundation.org, mgorman@techsingularity.net,
 aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com,
 liliang.opensource@gmail.com, virtio-dev@lists.oasis-open.org,
 yang.zhang.wz@gmail.com, quan.xu@aliyun.com
To: "Michael S. Tsirkin" <mst@redhat.com>
Return-path: <owner-linux-mm@kvack.org>
In-Reply-To: <20170712160129-mutt-send-email-mst@kernel.org>
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List-Id: kvm.vger.kernel.org

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



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