From: Wei Wang <wei.w.wang@intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
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
Subject: Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG
Date: Thu, 13 Jul 2017 15:42:35 +0800 [thread overview]
Message-ID: <5967246B.9030804@intel.com> (raw)
In-Reply-To: <20170712163746-mutt-send-email-mst@kernel.org>
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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next prev parent reply other threads:[~2017-07-13 7:40 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-12 12:40 [PATCH v12 0/8] Virtio-balloon Enhancement Wei Wang
2017-07-12 12:40 ` [PATCH v12 1/8] virtio-balloon: deflate via a page list Wei Wang
2017-07-12 12:40 ` [PATCH v12 2/8] virtio-balloon: coding format cleanup Wei Wang
2017-07-12 12:40 ` [PATCH v12 3/8] Introduce xbitmap Wei Wang
2017-07-12 12:40 ` [PATCH v12 4/8] xbitmap: add xb_find_next_bit() and xb_zero() Wei Wang
2017-07-12 12:40 ` [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG Wei Wang
2017-07-12 13:06 ` Michael S. Tsirkin
2017-07-12 13:29 ` Wei Wang
2017-07-12 13:56 ` Michael S. Tsirkin
2017-07-13 7:42 ` Wei Wang [this message]
2017-07-13 20:19 ` Michael S. Tsirkin
2017-07-14 7:12 ` Wei Wang
2017-07-23 1:45 ` Michael S. Tsirkin
2017-07-26 3:48 ` Wei Wang
2017-07-26 17:02 ` Michael S. Tsirkin
2017-07-27 2:50 ` Wei Wang
2017-07-28 23:08 ` Michael S. Tsirkin
2017-07-29 12:47 ` Wei Wang
2017-07-30 4:22 ` Michael S. Tsirkin
2017-07-30 5:59 ` Wang, Wei W
2017-07-30 16:18 ` Michael S. Tsirkin
2017-07-30 16:20 ` Michael S. Tsirkin
2017-07-31 12:36 ` Wei Wang
2017-07-13 0:44 ` Michael S. Tsirkin
2017-07-13 1:16 ` kbuild test robot
2017-07-13 4:21 ` kbuild test robot
2017-07-28 8:25 ` Wei Wang
2017-07-28 23:01 ` Michael S. Tsirkin
2017-07-12 12:40 ` [PATCH v12 6/8] mm: support reporting free page blocks Wei Wang
2017-07-13 0:33 ` Michael S. Tsirkin
2017-07-13 8:25 ` Wei Wang
2017-07-14 12:30 ` Michal Hocko
2017-07-14 12:54 ` Michal Hocko
2017-07-14 15:46 ` Michael S. Tsirkin
2017-07-14 19:17 ` Michael S. Tsirkin
2017-07-17 15:24 ` Michal Hocko
2017-07-18 2:12 ` Wei Wang
2017-07-19 8:13 ` Michal Hocko
2017-07-19 12:01 ` Wei Wang
2017-07-24 9:00 ` Michal Hocko
2017-07-25 9:32 ` Wei Wang
2017-07-25 11:25 ` Michal Hocko
2017-07-25 11:56 ` Wei Wang
2017-07-25 12:41 ` Michal Hocko
2017-07-25 14:47 ` Wang, Wei W
2017-07-25 14:53 ` Michal Hocko
2017-07-26 2:22 ` Wei Wang
2017-07-26 10:24 ` Michal Hocko
2017-07-26 11:44 ` Wei Wang
2017-07-26 11:55 ` Michal Hocko
2017-07-26 12:47 ` Wang, Wei W
2017-07-12 12:40 ` [PATCH v12 7/8] mm: export symbol of next_zone and first_online_pgdat Wei Wang
2017-07-13 0:16 ` Michael S. Tsirkin
2017-07-13 8:41 ` [virtio-dev] " Wei Wang
2017-07-14 12:31 ` Michal Hocko
2017-07-12 12:40 ` [PATCH v12 8/8] virtio-balloon: VIRTIO_BALLOON_F_CMD_VQ Wei Wang
2017-07-13 0:22 ` Michael S. Tsirkin
2017-07-13 8:46 ` Wei Wang
2017-07-13 17:59 ` Michael S. Tsirkin
2017-07-13 0:14 ` [PATCH v12 0/8] Virtio-balloon Enhancement Michael S. Tsirkin
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