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

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Message-ID: <5967246B.9030804@intel.com>
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