From: "Michael S. Tsirkin" <mst@redhat.com>
To: Wei Wang <wei.w.wang@intel.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 23:19:22 +0300 [thread overview]
Message-ID: <20170713210819-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <5967246B.9030804@intel.com>
On Thu, Jul 13, 2017 at 03:42:35PM +0800, Wei Wang wrote:
> 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.
And this only works if there are multiple rings like
avail + descriptor ring.
It won't work e.g. with the proposed new layout where
writing out a descriptor exposes it immediately.
> 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?
The way the new APIs poll used ring internally.
>
> > - some kind of iterator type would be nicer instead of
> > maintaining head/prev explicitly
>
> Why would we need to iterate the chain?
In your patches prev/tail are iterators - they keep track of
where you are in 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?
No - it means there's only 1 entry left for the last descriptor.
> 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
>
It is but let's make it explicit here - an API function is better
than a special value.
--
MST
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-07-13 20:19 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
2017-07-13 20:19 ` Michael S. Tsirkin [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170713210819-mutt-send-email-mst@kernel.org \
--to=mst@redhat.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=amit.shah@redhat.com \
--cc=cornelia.huck@de.ibm.com \
--cc=david@redhat.com \
--cc=kvm@vger.kernel.org \
--cc=liliang.opensource@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mgorman@techsingularity.net \
--cc=pbonzini@redhat.com \
--cc=qemu-devel@nongnu.org \
--cc=quan.xu@aliyun.com \
--cc=virtio-dev@lists.oasis-open.org \
--cc=virtualization@lists.linux-foundation.org \
--cc=wei.w.wang@intel.com \
--cc=yang.zhang.wz@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).