Date:   Thu, 27 Jul 2017 10:50:11 +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 07/27/2017 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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