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, 27 Jul 2017 10:50:11 +0800 [thread overview]
Message-ID: <597954E3.2070801@intel.com> (raw)
In-Reply-To: <20170726155856-mutt-send-email-mst@kernel.org>
On 07/27/2017 01:02 AM, Michael S. Tsirkin wrote:
> On Wed, Jul 26, 2017 at 11:48:41AM +0800, Wei Wang wrote:
>> On 07/23/2017 09:45 AM, Michael S. Tsirkin wrote:
>>> On Fri, Jul 14, 2017 at 03:12:43PM +0800, Wei Wang wrote:
>>>> On 07/14/2017 04:19 AM, Michael S. Tsirkin wrote:
>>>>> 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.
>>>> I think it can support the 1.1 proposal, too. But before getting
>>>> into that, I think we first need to deep dive into the implementation
>>>> and usage of _first/next/last. The usage would need to lock the vq
>>>> from the first to the end (otherwise, the returned info about the number
>>>> of available desc in the vq, i.e. num_free, would be invalid):
>>>>
>>>> lock(vq);
>>>> add_first();
>>>> add_next();
>>>> add_last();
>>>> unlock(vq);
>>>>
>>>> However, I think the case isn't this simple, since we need to check more
>>>> things
>>>> after each add_xx() step. For example, if only one entry is available at the
>>>> time
>>>> we start to use the vq, that is, num_free is 0 after add_first(), we
>>>> wouldn't be
>>>> able to add_next and add_last. So, it would work like this:
>>>>
>>>> start:
>>>> ...get free page block..
>>>> lock(vq)
>>>> retry:
>>>> ret = add_first(..,&num_free,);
>>>> if(ret == -ENOSPC) {
>>>> goto retry;
>>>> } else if (!num_free) {
>>>> add_chain_head();
>>>> unlock(vq);
>>>> kick & wait;
>>>> goto start;
>>>> }
>>>> next_one:
>>>> ...get free page block..
>>>> add_next(..,&num_free,);
>>>> if (!num_free) {
>>>> add_chain_head();
>>>> unlock(vq);
>>>> kick & wait;
>>>> goto start;
>>>> } if (num_free == 1) {
>>>> ...get free page block..
>>>> add_last(..);
>>>> unlock(vq);
>>>> kick & wait;
>>>> goto start;
>>>> } else {
>>>> goto next_one;
>>>> }
>>>>
>>>> The above seems unnecessary to me to have three different APIs.
>>>> That's the reason to combine them into one virtqueue_add_chain_desc().
>>>>
>>>> -- or, do you have a different thought about using the three APIs?
>>>>
>>>>
>>>> Implementation Reference:
>>>>
>>>> struct desc_iterator {
>>>> unsigned int head;
>>>> unsigned int tail;
>>>> };
>>>>
>>>> add_first(*vq, *desc_iterator, *num_free, ..)
>>>> {
>>>> if (vq->vq.num_free < 1)
>>>> return -ENOSPC;
>>>> get_desc(&desc_id);
>>>> desc[desc_id].flag &= ~_F_NEXT;
>>>> desc_iterator->head = desc_id
>>>> desc_iterator->tail = desc_iterator->head;
>>>> *num_free = vq->vq.num_free;
>>>> }
>>>>
>>>> add_next(vq, desc_iterator, *num_free,..)
>>>> {
>>>> get_desc(&desc_id);
>>>> desc[desc_id].flag &= ~_F_NEXT;
>>>> desc[desc_iterator.tail].next = desc_id;
>>>> desc[desc_iterator->tail].flag |= _F_NEXT;
>>>> desc_iterator->tail = desc_id;
>>>> *num_free = vq->vq.num_free;
>>>> }
>>>>
>>>> add_last(vq, desc_iterator,..)
>>>> {
>>>> get_desc(&desc_id);
>>>> desc[desc_id].flag &= ~_F_NEXT;
>>>> desc[desc_iterator.tail].next = desc_id;
>>>> desc_iterator->tail = desc_id;
>>>>
>>>> add_chain_head(); // put the desc_iterator.head to the ring
>>>> }
>>>>
>>>>
>>>> Best,
>>>> Wei
>>> OK I thought this over. While we might need these new APIs in
>>> the future, I think that at the moment, there's a way to implement
>>> this feature that is significantly simpler. Just add each s/g
>>> as a separate input buffer.
>>
>> Should it be an output buffer?
> Hypervisor overwrites these pages with zeroes. Therefore it is
> writeable by device: DMA_FROM_DEVICE.
Why would the hypervisor need to zero the buffer? I think it may only
need to read out the info(base,size).
I think it should be like this:
the cmd hdr buffer: input, because the hypervisor would write it to
send a cmd to the guest
the payload buffer: output, for the hypervisor to read the info
>> I think output means from the
>> driver to device (i.e. DMA_TO_DEVICE).
> This part is correct I believe.
>
>>> This needs zero new APIs.
>>>
>>> I know that follow-up patches need to add a header in front
>>> so you might be thinking: how am I going to add this
>>> header? The answer is quite simple - add it as a separate
>>> out header.
>>>
>>> Host will be able to distinguish between header and pages
>>> by looking at the direction, and - should we want to add
>>> IN data to header - additionally size (<4K => header).
>>
>> I think this works fine when the cmdq is only used for
>> reporting the unused pages.
>> It would be an issue
>> if there are other usages (e.g. report memory statistics)
>> interleaving. I think one solution would be to lock the cmdq until
>> a cmd usage is done ((e.g. all the unused pages are reported) ) -
>> in this case, the periodically updated guest memory statistics
>> may be delayed for a while occasionally when live migration starts.
>> Would this be acceptable? If not, probably we can have the cmdq
>> for one usage only.
>>
>>
>> Best,
>> Wei
> OK I see, I think the issue is that reporting free pages
> was structured like stats. Let's split it -
> send pages on e.g. free_vq, get commands on vq shared with
> stats.
>
Would it be better to have the "report free page" command to be sent
through the free_vq? In this case,we will have
stats_vq: for the stats usage, which is already there
free_vq: for reporting free pages.
Best,
Wei
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next prev parent reply other threads:[~2017-07-27 2:47 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
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 [this message]
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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