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Date: Thu, 13 Jul 2017 15:42:35 +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 09:56 PM, Michael S. Tsirkin wrote: > > So the way I see it, there are several issues: > > - internal wait - forces multiple APIs like kick/kick_sync > note how kick_sync can fail but your code never checks return code > - need to re-write the last descriptor - might not work > for alternative layouts which always expose descriptors > immediately Probably it wasn't clear. Please let me explain the two functions here: 1) virtqueue_add_chain_desc(vq, head_id, prev_id,..): grabs a desc from the vq and inserts it to the chain tail (which is indexed by prev_id, probably better to call it tail_id). Then, the new added desc becomes the tail (i.e. the last desc). The _F_NEXT flag is cleared for each desc when it's added to the chain, and set when another desc comes to follow later. 2) virtqueue_add_chain(vq, head_id,..): expose the chain to the other end. So, if people want to add a desc and immediately expose it to the other end, i.e. build a single desc chain, they can just add and expose: virtqueue_add_chain_desc(..); virtqueue_add_chain(..,head_id); Would you see any issues here? > - some kind of iterator type would be nicer instead of > maintaining head/prev explicitly Why would we need to iterate the chain? I think it would be simpler to use a wrapper struct: struct virtqueue_desc_chain { unsigned int head; // head desc id of the chain unsigned int tail; // tail desc id of the chain } The new desc will be put to desc[tail].next, and we don't need to walk from the head desc[head].next when inserting a new desc to the chain, right? > > As for the use, it would be better to do > > if (!add_next(vq, ...)) { > add_last(vq, ...) > kick > wait > } "!add_next(vq, ...)" means that the vq is full? If so, what would add_last() do then? > Using VIRTQUEUE_DESC_ID_INIT seems to avoid a branch in the driver, but > in fact it merely puts the branch in the virtio code. > Actually it wasn't intended to improve performance. It is used to indicate the "init" state of the chain. So, when virtqueue_add_chain_desc(, head_id,..) finds head id=INIT, it will assign the grabbed desc id to &head_id. In some sense, it is equivalent to add_first(). Do you have a different opinion here? Best, Wei
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