Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC PATCH 00/22] Enhance VHOST to enable SoC-to-SoC
 communication
To:     Kishon Vijay Abraham I <kishon@ti.com>,
        Cornelia Huck <cohuck@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Ohad Ben-Cohen <ohad@wizery.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Jon Mason <jdmason@kudzu.us>,
        Dave Jiang <dave.jiang@intel.com>,
        Allen Hubbe <allenbh@gmail.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Stefano Garzarella <sgarzare@redhat.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-remoteproc@vger.kernel.org, linux-ntb@googlegroups.com,
        linux-pci@vger.kernel.org, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org
References: <20200702082143.25259-1-kishon@ti.com>
 <20200702055026-mutt-send-email-mst@kernel.org>
 <603970f5-3289-cd53-82a9-aa62b292c552@redhat.com>
 <14c6cad7-9361-7fa4-e1c6-715ccc7e5f6b@ti.com>
 <59fd6a0b-8566-44b7-3dae-bb52b468219b@redhat.com>
 <ce9eb6a5-cd3a-a390-5684-525827b30f64@ti.com>
 <da2b671c-b05d-a57f-7bdf-8b1043a41240@redhat.com>
 <fee8a0fb-f862-03bd-5ede-8f105b6af529@ti.com>
 <b2178e1d-2f5c-e8a3-72fb-70f2f8d6aa45@redhat.com>
 <45a8a97c-2061-13ee-5da8-9877a4a3b8aa@ti.com>
 <c8739d7f-e12e-f6a2-7018-9eeaf6feb054@redhat.com>
 <20200828123409.4cd2a812.cohuck@redhat.com>
 <ac8f7e4f-9f46-919a-f5c2-89b07794f0ab@ti.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <9cd58cd1-0041-3d98-baf7-6e5bc2e7e317@redhat.com>
Date:   Tue, 1 Sep 2020 16:50:03 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
In-Reply-To: <ac8f7e4f-9f46-919a-f5c2-89b07794f0ab@ti.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2020/9/1 下午1:24, Kishon Vijay Abraham I wrote:
> Hi,
>
> On 28/08/20 4:04 pm, Cornelia Huck wrote:
>> On Thu, 9 Jul 2020 14:26:53 +0800
>> Jason Wang <jasowang@redhat.com> wrote:
>>
>> [Let me note right at the beginning that I first noted this while
>> listening to Kishon's talk at LPC on Wednesday. I might be very
>> confused about the background here, so let me apologize beforehand for
>> any confusion I might spread.]
>>
>>> On 2020/7/8 下午9:13, Kishon Vijay Abraham I wrote:
>>>> Hi Jason,
>>>>
>>>> On 7/8/2020 4:52 PM, Jason Wang wrote:
>>>>> On 2020/7/7 下午10:45, Kishon Vijay Abraham I wrote:
>>>>>> Hi Jason,
>>>>>>
>>>>>> On 7/7/2020 3:17 PM, Jason Wang wrote:
>>>>>>> On 2020/7/6 下午5:32, Kishon Vijay Abraham I wrote:
>>>>>>>> Hi Jason,
>>>>>>>>
>>>>>>>> On 7/3/2020 12:46 PM, Jason Wang wrote:
>>>>>>>>> On 2020/7/2 下午9:35, Kishon Vijay Abraham I wrote:
>>>>>>>>>> Hi Jason,
>>>>>>>>>>
>>>>>>>>>> On 7/2/2020 3:40 PM, Jason Wang wrote:
>>>>>>>>>>> On 2020/7/2 下午5:51, Michael S. Tsirkin wrote:
>>>>>>>>>>>> On Thu, Jul 02, 2020 at 01:51:21PM +0530, Kishon Vijay 
>>>>>>>>>>>> Abraham I wrote:
>>>>>>>>>>>>> This series enhances Linux Vhost support to enable SoC-to-SoC
>>>>>>>>>>>>> communication over MMIO. This series enables rpmsg 
>>>>>>>>>>>>> communication between
>>>>>>>>>>>>> two SoCs using both PCIe RC<->EP and HOST1-NTB-HOST2
>>>>>>>>>>>>>
>>>>>>>>>>>>> 1) Modify vhost to use standard Linux driver model
>>>>>>>>>>>>> 2) Add support in vring to access virtqueue over MMIO
>>>>>>>>>>>>> 3) Add vhost client driver for rpmsg
>>>>>>>>>>>>> 4) Add PCIe RC driver (uses virtio) and PCIe EP driver 
>>>>>>>>>>>>> (uses vhost) for
>>>>>>>>>>>>>          rpmsg communication between two SoCs connected to 
>>>>>>>>>>>>> each other
>>>>>>>>>>>>> 5) Add NTB Virtio driver and NTB Vhost driver for rpmsg 
>>>>>>>>>>>>> communication
>>>>>>>>>>>>>          between two SoCs connected via NTB
>>>>>>>>>>>>> 6) Add configfs to configure the components
>>>>>>>>>>>>>
>>>>>>>>>>>>> UseCase1 :
>>>>>>>>>>>>>
>>>>>>>>>>>>>        VHOST RPMSG VIRTIO RPMSG
>>>>>>>>>>>>> +                               +
>>>>>>>>>>>>> |                               |
>>>>>>>>>>>>> |                               |
>>>>>>>>>>>>> |                               |
>>>>>>>>>>>>> |                               |
>>>>>>>>>>>>> +-----v------+ +------v-------+
>>>>>>>>>>>>> |   Linux    |                 | Linux    |
>>>>>>>>>>>>> |  Endpoint  |                 | Root Complex |
>>>>>>>>>>>>> | <----------------->              |
>>>>>>>>>>>>> |            | |              |
>>>>>>>>>>>>> |    SOC1    |                 | SOC2     |
>>>>>>>>>>>>> +------------+ +--------------+
>>>>>>>>>>>>>
>>>>>>>>>>>>> UseCase 2:
>>>>>>>>>>>>>
>>>>>>>>>>>>>            VHOST RPMSG VIRTIO RPMSG
>>>>>>>>>>>>> + +
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> +------v------+ +------v------+
>>>>>>>>>>>>>          | | |             |
>>>>>>>>>>>>>          |    HOST1 |                                   | 
>>>>>>>>>>>>> HOST2    |
>>>>>>>>>>>>>          | | |             |
>>>>>>>>>>>>> +------^------+ +------^------+
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> | |
>>>>>>>>>>>>> +---------------------------------------------------------------------+ 
>>>>>>>>>>>>>
>>>>>>>>>>>>> | +------v------+ +------v------+  |
>>>>>>>>>>>>> |  | | |             |  |
>>>>>>>>>>>>> |  |     EP |                                   | EP      
>>>>>>>>>>>>> |  |
>>>>>>>>>>>>> |  | CONTROLLER1 |                                   | 
>>>>>>>>>>>>> CONTROLLER2 |  |
>>>>>>>>>>>>> |  | <-----------------------------------> |  |
>>>>>>>>>>>>> |  | | |             |  |
>>>>>>>>>>>>> |  | | |             |  |
>>>>>>>>>>>>> |  |             |  SoC With Multiple EP Instances   
>>>>>>>>>>>>> |             |  |
>>>>>>>>>>>>> |  |             |  (Configured using NTB Function)  
>>>>>>>>>>>>> |             |  |
>>>>>>>>>>>>> | +-------------+ +-------------+  |
>>>>>>>>>>>>> +---------------------------------------------------------------------+ 
>>>>>>>>>>>>>
>>
>> First of all, to clarify the terminology:
>> Is "vhost rpmsg" acting as what the virtio standard calls the 'device',
>> and "virtio rpmsg" as the 'driver'? Or is the "vhost" part mostly just
>
> Right, vhost_rpmsg is 'device' and virtio_rpmsg is 'driver'.
>> virtqueues + the exiting vhost interfaces?
>
> It's implemented to provide the full 'device' functionality.
>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Software Layering:
>>>>>>>>>>>>>
>>>>>>>>>>>>> The high-level SW layering should look something like 
>>>>>>>>>>>>> below. This series
>>>>>>>>>>>>> adds support only for RPMSG VHOST, however something 
>>>>>>>>>>>>> similar should be
>>>>>>>>>>>>> done for net and scsi. With that any vhost device (PCI, 
>>>>>>>>>>>>> NTB, Platform
>>>>>>>>>>>>> device, user) can use any of the vhost client driver.
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>           +----------------+ +-----------+  +------------+ 
>>>>>>>>>>>>> +----------+
>>>>>>>>>>>>>           |  RPMSG VHOST   |  | NET VHOST |  | SCSI VHOST 
>>>>>>>>>>>>> |  |    X     |
>>>>>>>>>>>>>           +-------^--------+ +-----^-----+  +-----^------+ 
>>>>>>>>>>>>> +----^-----+
>>>>>>>>>>>>>                   | |              |              |
>>>>>>>>>>>>>                   | |              |              |
>>>>>>>>>>>>>                   | |              |              |
>>>>>>>>>>>>> +-----------v-----------------v--------------v--------------v----------+ 
>>>>>>>>>>>>>
>>>>>>>>>>>>> |                            VHOST 
>>>>>>>>>>>>> CORE                                |
>>>>>>>>>>>>> +--------^---------------^--------------------^------------------^-----+ 
>>>>>>>>>>>>>
>>>>>>>>>>>>>                | |                    |                  |
>>>>>>>>>>>>>                | |                    |                  |
>>>>>>>>>>>>>                | |                    |                  |
>>>>>>>>>>>>> +--------v-------+  +----v------+ +----------v----------+  
>>>>>>>>>>>>> +----v-----+
>>>>>>>>>>>>> |  PCI EPF VHOST |  | NTB VHOST | |PLATFORM DEVICE VHOST|  
>>>>>>>>>>>>> |    X     |
>>>>>>>>>>>>> +----------------+  +-----------+ +---------------------+  
>>>>>>>>>>>>> +----------+
>>
>> So, the upper half is basically various functionality types, e.g. a net
>> device. What is the lower half, a hardware interface? Would it be
>> equivalent to e.g. a normal PCI device?
>
> Right, the upper half should provide the functionality.
> The bottom layer could be a HW interface (like PCIe device or NTB 
> device) or it could be a SW interface (for accessing virtio ring in 
> userspace) that could be used by Hypervisor.
>
> The top half should be transparent to what type of device is actually 
> using it.
>
>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> This was initially proposed here [1]
>>>>>>>>>>>>>
>>>>>>>>>>>>> [1] ->
>>>>>>>>>>>>> https://lore.kernel.org/r/2cf00ec4-1ed6-f66e-6897-006d1a5b6390@ti.com 
>>>>>>>>>>>>>
>>>>>>>>>>>> I find this very interesting. A huge patchset so will take 
>>>>>>>>>>>> a bit
>>>>>>>>>>>> to review, but I certainly plan to do that. Thanks!
>>>>>>>>>>> Yes, it would be better if there's a git branch for us to 
>>>>>>>>>>> have a look.
>>>>>>>>>> I've pushed the branch
>>>>>>>>>> https://github.com/kishon/linux-wip.git vhost_rpmsg_pci_ntb_rfc
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>>> Btw, I'm not sure I get the big picture, but I vaguely feel 
>>>>>>>>>>> some of the
>>>>>>>>>>> work is
>>>>>>>>>>> duplicated with vDPA (e.g the epf transport or vhost bus).
>>>>>>>>>> This is about connecting two different HW systems both 
>>>>>>>>>> running Linux and
>>>>>>>>>> doesn't necessarily involve virtualization.
>>>>>>>>> Right, this is something similar to VOP
>>>>>>>>> (Documentation/misc-devices/mic/mic_overview.rst). The 
>>>>>>>>> different is the
>>>>>>>>> hardware I guess and VOP use userspace application to 
>>>>>>>>> implement the device.
>>>>>>>> I'd also like to point out, this series tries to have 
>>>>>>>> communication between
>>>>>>>> two
>>>>>>>> SoCs in vendor agnostic way. Since this series solves for 2 
>>>>>>>> usecases (PCIe
>>>>>>>> RC<->EP and NTB), for the NTB case it directly plugs into NTB 
>>>>>>>> framework and
>>>>>>>> any
>>>>>>>> of the HW in NTB below should be able to use a virtio-vhost 
>>>>>>>> communication
>>>>>>>>
>>>>>>>> #ls drivers/ntb/hw/
>>>>>>>> amd  epf  idt  intel  mscc
>>>>>>>>
>>>>>>>> And similarly for the PCIe RC<->EP communication, this adds a 
>>>>>>>> generic endpoint
>>>>>>>> function driver and hence any SoC that supports configurable 
>>>>>>>> PCIe endpoint can
>>>>>>>> use virtio-vhost communication
>>>>>>>>
>>>>>>>> # ls drivers/pci/controller/dwc/*ep*
>>>>>>>> drivers/pci/controller/dwc/pcie-designware-ep.c
>>>>>>>> drivers/pci/controller/dwc/pcie-uniphier-ep.c
>>>>>>>> drivers/pci/controller/dwc/pci-layerscape-ep.c
>>>>>>> Thanks for those backgrounds.
>>>>>>>
>>>>>>>>>>       So there is no guest or host as in
>>>>>>>>>> virtualization but two entirely different systems connected 
>>>>>>>>>> via PCIe cable,
>>>>>>>>>> one
>>>>>>>>>> acting as guest and one as host. So one system will provide 
>>>>>>>>>> virtio
>>>>>>>>>> functionality reserving memory for virtqueues and the other 
>>>>>>>>>> provides vhost
>>>>>>>>>> functionality providing a way to access the virtqueues in 
>>>>>>>>>> virtio memory.
>>>>>>>>>> One is
>>>>>>>>>> source and the other is sink and there is no intermediate 
>>>>>>>>>> entity. (vhost was
>>>>>>>>>> probably intermediate entity in virtualization?)
>>>>>>>>> (Not a native English speaker) but "vhost" could introduce 
>>>>>>>>> some confusion for
>>>>>>>>> me since it was use for implementing virtio backend for 
>>>>>>>>> userspace drivers. I
>>>>>>>>> guess "vringh" could be better.
>>>>>>>> Initially I had named this vringh but later decided to choose 
>>>>>>>> vhost instead of
>>>>>>>> vringh. vhost is still a virtio backend (not necessarily 
>>>>>>>> userspace) though it
>>>>>>>> now resides in an entirely different system. Whatever virtio is 
>>>>>>>> for a frontend
>>>>>>>> system, vhost can be that for a backend system. vring can be 
>>>>>>>> for accessing
>>>>>>>> virtqueue and can be used either in frontend or backend.
>>
>> I guess that clears up at least some of my questions from above...
>>
>>>>>>> Ok.
>>>>>>>
>>>>>>>>>>> Have you considered to implement these through vDPA?
>>>>>>>>>> IIUC vDPA only provides an interface to userspace and an 
>>>>>>>>>> in-kernel rpmsg
>>>>>>>>>> driver
>>>>>>>>>> or vhost net driver is not provided.
>>>>>>>>>>
>>>>>>>>>> The HW connection looks something like 
>>>>>>>>>> https://pasteboard.co/JfMVVHC.jpg
>>>>>>>>>> (usecase2 above),
>>>>>>>>> I see.
>>>>>>>>>
>>>>>>>>>>       all the boards run Linux. The middle board provides NTB
>>>>>>>>>> functionality and board on either side provides virtio/vhost
>>>>>>>>>> functionality and
>>>>>>>>>> transfer data using rpmsg.
>>
>> This setup looks really interesting (sometimes, it's really hard to
>> imagine this in the abstract.)
>>>>>>>>> So I wonder whether it's worthwhile for a new bus. Can we use
>>>>>>>>> the existed virtio-bus/drivers? It might work as, except for
>>>>>>>>> the epf transport, we can introduce a epf "vhost" transport
>>>>>>>>> driver.
>>>>>>>> IMHO we'll need two buses one for frontend and other for
>>>>>>>> backend because the two components can then co-operate/interact
>>>>>>>> with each other to provide a functionality. Though both will
>>>>>>>> seemingly provide similar callbacks, they are both provide
>>>>>>>> symmetrical or complimentary funcitonality and need not be same
>>>>>>>> or identical.
>>>>>>>>
>>>>>>>> Having the same bus can also create sequencing issues.
>>>>>>>>
>>>>>>>> If you look at virtio_dev_probe() of virtio_bus
>>>>>>>>
>>>>>>>> device_features = dev->config->get_features(dev);
>>>>>>>>
>>>>>>>> Now if we use same bus for both front-end and back-end, both
>>>>>>>> will try to get_features when there has been no set_features.
>>>>>>>> Ideally vhost device should be initialized first with the set
>>>>>>>> of features it supports. Vhost and virtio should use "status"
>>>>>>>> and "features" complimentarily and not identically.
>>>>>>> Yes, but there's no need for doing status/features passthrough
>>>>>>> in epf vhost drivers.b
>>>>>>>
>>>>>>>> virtio device (or frontend) cannot be initialized before vhost
>>>>>>>> device (or backend) gets initialized with data such as
>>>>>>>> features. Similarly vhost (backend)
>>>>>>>> cannot access virqueues or buffers before virtio (frontend) sets
>>>>>>>> VIRTIO_CONFIG_S_DRIVER_OK whereas that requirement is not there
>>>>>>>> for virtio as the physical memory for virtqueues are created by
>>>>>>>> virtio (frontend).
>>>>>>> epf vhost drivers need to implement two devices: vhost(vringh)
>>>>>>> device and virtio device (which is a mediated device). The
>>>>>>> vhost(vringh) device is doing feature negotiation with the
>>>>>>> virtio device via RC/EP or NTB. The virtio device is doing
>>>>>>> feature negotiation with local virtio drivers. If there're
>>>>>>> feature mismatch, epf vhost drivers and do mediation between
>>>>>>> them.
>>>>>> Here epf vhost should be initialized with a set of features for
>>>>>> it to negotiate either as vhost device or virtio device no? Where
>>>>>> should the initial feature set for epf vhost come from?
>>>>>
>>>>> I think it can work as:
>>>>>
>>>>> 1) Having an initial features (hard coded in the code) set X in
>>>>> epf vhost 2) Using this X for both virtio device and vhost(vringh)
>>>>> device 3) local virtio driver will negotiate with virtio device
>>>>> with feature set Y 4) remote virtio driver will negotiate with
>>>>> vringh device with feature set Z 5) mediate between feature Y and
>>>>> feature Z since both Y and Z are a subset of X
>>>>>
>>>> okay. I'm also thinking if we could have configfs for configuring
>>>> this. Anyways we could find different approaches of configuring
>>>> this.
>>>
>>>
>>> Yes, and I think some management API is needed even in the design of
>>> your "Software Layering". In that figure, rpmsg vhost need some
>>> pre-set or hard-coded features.
>>
>> When I saw the plumbers talk, my first idea was "this needs to be a new
>> transport". You have some hard-coded or pre-configured features, and
>> then features are negotiated via a transport-specific means in the
>> usual way. There's basically an extra/extended layer for this (and
>> status, and whatever).
>
> I think for PCIe root complex to PCIe endpoint communication it's 
> still "Virtio Over PCI Bus", though existing layout cannot be used in 
> this context (find virtio capability will fail for modern interface 
> and loading queue status immediately after writing queue number is not 
> possible for root complex to endpoint communication; setup_vq() in 
> virtio_pci_legacy.c).


Then you need something that is functional equivalent to virtio PCI 
which is actually the concept of vDPA (e.g vDPA provides alternatives if 
the queue_sel is hard in the EP implementation).


>
> "Virtio Over NTB" should anyways be a new transport.
>>
>> Does that make any sense?
>
> yeah, in the approach I used the initial features are hard-coded in 
> vhost-rpmsg (inherent to the rpmsg) but when we have to use adapter 
> layer (vhost only for accessing virtio ring and use virtio drivers on 
> both front end and backend), based on the functionality (e.g, rpmsg), 
> the vhost should be configured with features (to be presented to the 
> virtio) and that's why additional layer or APIs will be required.


A question here, if we go with vhost bus approach, does it mean the 
virtio device can only be implemented in EP's userspace?

Thanks


>>
>>>
>>>
>>>>>>>>> It will have virtqueues but only used for the communication
>>>>>>>>> between itself and
>>>>>>>>> uppter virtio driver. And it will have vringh queues which
>>>>>>>>> will be probe by virtio epf transport drivers. And it needs to
>>>>>>>>> do datacopy between virtqueue and
>>>>>>>>> vringh queues.
>>>>>>>>>
>>>>>>>>> It works like:
>>>>>>>>>
>>>>>>>>> virtio drivers <- virtqueue/virtio-bus -> epf vhost drivers <-
>>>>>>>>> vringh queue/epf>
>>>>>>>>>
>>>>>>>>> The advantages is that there's no need for writing new buses
>>>>>>>>> and drivers.
>>>>>>>> I think this will work however there is an addtional copy
>>>>>>>> between vringh queue and virtqueue,
>>>>>>> I think not? E.g in use case 1), if we stick to virtio bus, we
>>>>>>> will have:
>>>>>>>
>>>>>>> virtio-rpmsg (EP) <- virtio ring(1) -> epf vhost driver (EP) <-
>>>>>>> virtio ring(2) -> virtio pci (RC) <-> virtio rpmsg (RC)
>>>>>> IIUC epf vhost driver (EP) will access virtio ring(2) using
>>>>>> vringh?
>>>>>
>>>>> Yes.
>>>>>
>>>>>> And virtio
>>>>>> ring(2) is created by virtio pci (RC).
>>>>>
>>>>> Yes.
>>>>>
>>>>>>> What epf vhost driver did is to read from virtio ring(1) about
>>>>>>> the buffer len and addr and them DMA to Linux(RC)?
>>>>>> okay, I made some optimization here where vhost-rpmsg using a
>>>>>> helper writes a buffer from rpmsg's upper layer directly to
>>>>>> remote Linux (RC) as against here were it has to be first written
>>>>>> to virtio ring (1).
>>>>>>
>>>>>> Thinking how this would look for NTB
>>>>>> virtio-rpmsg (HOST1) <- virtio ring(1) -> NTB(HOST1) <->
>>>>>> NTB(HOST2)  <- virtio ring(2) -> virtio-rpmsg (HOST2)
>>>>>>
>>>>>> Here the NTB(HOST1) will access the virtio ring(2) using vringh?
>>>>>
>>>>> Yes, I think so it needs to use vring to access virtio ring (1) as
>>>>> well.
>>>> NTB(HOST1) and virtio ring(1) will be in the same system. So it
>>>> doesn't have to use vring. virtio ring(1) is by the virtio device
>>>> the NTB(HOST1) creates.
>>>
>>>
>>> Right.
>>>
>>>
>>>>>> Do you also think this will work seamlessly with virtio_net.c,
>>>>>> virtio_blk.c?
>>>>>
>>>>> Yes.
>>>> okay, I haven't looked at this but the backend of virtio_blk should
>>>> access an actual storage device no?
>>>
>>>
>>> Good point, for non-peer device like storage. There's probably no
>>> need for it to be registered on the virtio bus and it might be better
>>> to behave as you proposed.
>>
>> I might be missing something; but if you expose something as a block
>> device, it should have something it can access with block reads/writes,
>> shouldn't it? Of course, that can be a variety of things.
>>
>>>
>>> Just to make sure I understand the design, how is VHOST SCSI expected
>>> to work in your proposal, does it have a device for file as a backend?
>>>
>>>
>>>>>> I'd like to get clarity on two things in the approach you
>>>>>> suggested, one is features (since epf vhost should ideally be
>>>>>> transparent to any virtio driver)
>>>>>
>>>>> We can have have an array of pre-defined features indexed by
>>>>> virtio device id in the code.
>>>>>
>>>>>> and the other is how certain inputs to virtio device such as
>>>>>> number of buffers be determined.
>>>>>
>>>>> We can start from hard coded the value like 256, or introduce some
>>>>> API for user to change the value.
>>>>>
>>>>>> Thanks again for your suggestions!
>>>>>
>>>>> You're welcome.
>>>>>
>>>>> Note that I just want to check whether or not we can reuse the
>>>>> virtio bus/driver. It's something similar to what you proposed in
>>>>> Software Layering but we just replace "vhost core" with "virtio
>>>>> bus" and move the vhost core below epf/ntb/platform transport.
>>>> Got it. My initial design was based on my understanding of your
>>>> comments [1].
>>>
>>>
>>> Yes, but that's just for a networking device. If we want something
>>> more generic, it may require more thought (bus etc).
>>
>> I believe that we indeed need something bus-like to be able to support
>> a variety of devices.
>
> I think we could still have adapter layers for different types of 
> devices ([1]) and use existing virtio bus for both front end and back 
> end. Using bus-like will however simplify adding support for new types 
> of devices and adding adapters for devices will be slightly more complex.
>
> [1] -> Page 13 in 
> https://linuxplumbersconf.org/event/7/contributions/849/attachments/642/1175/Virtio_for_PCIe_RC_EP_NTB.pdf
>>
>>>
>>>
>>>>
>>>> I'll try to create something based on your proposed design here.
>>>
>>>
>>> Sure, but for coding, we'd better wait for other's opinion here.
>>
>> Please tell me if my thoughts above make any sense... I have just
>> started looking at that, so I might be completely off.
>
> I think your understanding is correct! Thanks for your inputs.
>
> Thanks
> Kishon