Re: [virtio-dev] [PATCH 0/2] introduce virtio-ism: internal shared memory device

[Tony Lu <tonylu@linux.alibaba.com>]

On Wed, Oct 19, 2022 at 05:04:58PM +0800, Jason Wang wrote:
> 
> 在 2022/10/19 16:07, Tony Lu 写道:
> > On Wed, Oct 19, 2022 at 02:02:21PM +0800, Xuan Zhuo wrote:
> > > On Wed, 19 Oct 2022 12:36:35 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > On Wed, Oct 19, 2022 at 12:22 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > > On Wed, 19 Oct 2022 11:56:52 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > > On Wed, Oct 19, 2022 at 10:42 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> > > > > > > On Mon, 17 Oct 2022 16:17:31 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > > > > > 
> > > > > > > 
> > > > > > > Hi Jason,
> > > > > > > 
> > > > > > > I think there may be some problems with the direction we are discussing.
> > > > > > Probably not.
> > > > > > 
> > > > > > As far as we are focusing on technology, there's nothing wrong from my
> > > > > > perspective. And this is how the community works. Your idea needs to
> > > > > > be justified and people are free to raise any technical questions
> > > > > > especially considering you've posted a spec change with prototype
> > > > > > codes but not only the idea.
> > > > > > 
> > > > > > > Our
> > > > > > > goal is to add an new ism device. As far as the spec is concerned, we are not
> > > > > > > concerned with the implementation of the backend.
> > > > > > > 
> > > > > > > The direction we should discuss is what is the difference between the ism device
> > > > > > > and other devices such as virtio-net, and whether it is necessary to introduce
> > > > > > > this new device.
> > > > > > This is somehow what I want to ask, actually it's not a comparison
> > > > > > with virtio-net but:
> > > > > > 
> > > > > > - virtio-roce
> > > > > > - virtio-vhost-user
> > > > > > - virtio-(p)mem
> > > > > > 
> > > > > > or whether we can simply add features to those devices to achieve what
> > > > > > you want to do here.
> > > > > 
> > > > > Yes, this is my priority to discuss.
> > > > > 
> > > > > At the moment, I think the most similar to ism is the Vhost-user Device Backend
> > > > > of virtio-vhost-user.
> > > > > 
> > > > > My understanding of it is to map any virtio device to another vm as a vvu
> > > > > device.
> > > > Yes, so a possible way is to have a device with memory zone/region
> > > > provision and management then map it via virtio-vhost-user.
> > > 
> > > Yes, there is such a possibility. virtio-vhost-user makes me feel that what can
> > > be shared is the function implementation of map.
> > > 
> > > But in the vm to provide the interface to the upper layer, I think this is the
> > > work of ism.
> > > 
> > > But one of the reasons why I didn't use virtio-vhost-user directly is that in
> > > another vm, the guest can operate the vvu device, which we hope that both sides
> > > are equal to the ism device.
> > > 
> > > So I want to agree on a question first: who will provide the upper layer with
> > > the ability to share the memory area?
> > > 
> > > Our answer is a new ism device. How does this device achieve memory sharing, I
> > > think is the second question.
> > > 
> > > 
> > > > >  From this design purpose, I think the two are different.
> > > > > 
> > > > > Of course, you might want to extend it, it does have some similarities and uses
> > > > > a lot of similar techniques.
> > > > I don't have any preference so far. If you think your idea makes more
> > > > sense, then try your best to justify it in the list.
> > > > 
> > > > > So we can really discuss in this direction, whether
> > > > > the vvu device can be extended to achieve the purpose of ism, or whether the
> > > > > design goals can be agreed.
> > > > I've added Stefan in the loop, let's hear from him.
> > > > 
> > > > > Or, in the direction of memory sharing in the backend, can ism and vvu be merged?
> > > > > Should device/driver APIs remain independent?
> > > > Btw, you mentioned that one possible user of ism is the smc, but I
> > > > don't see how it connects to that with your prototype driver.
> > > Yes, we originally had plans, but the virtio spec was considered for submission,
> > > so this was not included. Maybe, we should have included this part @Tony
> > > 
> > > A brief introduction is that SMC currently has a corresponding
> > > s390/net/ism_drv.c and we will replace this in the virtualization scenario.
> 
> 
> Ok, I see. So I think the goal is to implement something in virtio that is
> functional equivalent to IBM ISM device.
> 

Yes, IBM ISM devices do something similar and it inspired this.

> 
> > > 
> > > Thanks.
> > > 
> > SMC is a network protocol which is modeled by shared memory rather than
> > packet.
> 
> 
> After reading more SMC from IBM website, I think you meant SMC-D here. And I
> wonder in order to have a complete SMC solution we still need virtio-ROCE
> for inter host communcation?
> 

Mostly yes.

SMC-D is the part of whole SMC solution. SMC supports multiple
underlying device, -D means ISM device, -R means RDMA device. The key
data model is shared memory, SMC uses RDMA (-R) or ISM(-D) to *share*
memory between peers, and it will choose the suitable device on demand
during handshaking. If there was no suitable device, it would fall back
to TCP. So virtio-ROCE is not required.

> 
> >   Actually the basic required interfaces of SMC device are:
> > 
> >    - alloc / free memory region, each connection peer has two memory
> > 	regions dynamically for sending and receiving ring buffer.
> >    - attach / detach memory region, remote attaches local-allocated
> > 	sending region as receiving region, vice versa.
> >    - notify, tell peer to read data and update cursor.
> > 
> > Then the device can be registered as SMC ISM device. Of course, SMC
> > also requires some modification to adapt it.
> 
> 
> Looking at s390 ism driver it requires other stuffs like vlan add/remove or
> gid query, do we need them as well?

vlan is not required in this use case. ISM uses gid to identified each
others, maybe we could implement it in virtio ways.

To support virtio-ism smoothly, the interfaces of ISM driver still need
to be adjusted. I will put it on the table with IBM people.

Cheers,
Tony Lu

> 
> Thanks
> 
> 
> > 
> > Cheers,
> > Tony Lu
> > 
> > > > Thanks
> > > > 
> > > > > Thanks.
> > > > > 
> > > > > 
> > > > > > > How to share the backend with other deivce is another problem.
> > > > > > Yes, anything that is used for your virito-ism prototype can be used
> > > > > > for other devices.
> > > > > > 
> > > > > > > Our goal is to dynamically obtain a piece of memory to share with other vms.
> > > > > > So at this level, I don't see the exact difference compared to
> > > > > > virtio-vhost-user. Let's just focus on the API that carries on the
> > > > > > semantic:
> > > > > > 
> > > > > > - map/unmap
> > > > > > - permission update
> > > > > > 
> > > > > > The only missing piece is the per region notification.
> > > > > > 
> > > > > > > In a connection, this memory will be used repeatedly. As far as SMC is concerned,
> > > > > > > it will use it as a ring. Of course, we also need a notify mechanism.
> > > > > > > 
> > > > > > > That's what we're aiming for, so we should first discuss whether this
> > > > > > > requirement is reasonable.
> > > > > > So unless somebody said "no", it is fine until now.
> > > > > > 
> > > > > > > I think it's a feature currently not supported by
> > > > > > > other devices specified by the current virtio spce.
> > > > > > Probably, but we've already had rfcs for roce and vhost-user.
> > > > > > 
> > > > > > Thanks
> > > > > > 
> > > > > > > Thanks.
> > > > > > > 
> > > > > > >