Re: [PATCH 2/5] Introduce VIRTIO_F_ADMIN_VQ_INDIRECT_DESC/VIRTIO_F_ADMIN_VQ_IN_ORDER

[Jason Wang <jasowang@redhat.com>]

On Wed, Jan 26, 2022 at 5:27 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
>
> On 1/26/2022 9:03 AM, Jason Wang wrote:
> > On Tue, Jan 25, 2022 at 6:59 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
> >>
> >> On 1/25/2022 5:52 AM, Parav Pandit wrote:
> >>> Hi Jason,
> >>>
> >>>> From: Jason Wang <jasowang@redhat.com>
> >>>> Sent: Tuesday, January 25, 2022 8:59 AM
> >>>>
> >>>> å 2022/1/19 äå12:48, Parav Pandit åé:
> >>>>>> From: Jason Wang <jasowang@redhat.com>
> >>>>>> Sent: Wednesday, January 19, 2022 9:33 AM
> >>>>>>
> >>>>>>
> >>>>>> It it means IMS, there's already a proposal[1] that introduce MSI
> >>>>>> commands via the admin virtqueue. And we had similar requirement for
> >>>>>> virtio-MMIO[2] and managed device or SF [3], so I would rather to
> >>>>>> introduce IMS (need a better name though) as a basic facility instead
> >>>>>> of tie it to any specific transport.
> >>>>>>
> >>>>> IMS of [1] is a interrupt configuration by the virtio driver for the device is it
> >>>> driving, which needs a queue.
> >>>>> So regardless of the device type as PCI PF/VF/SF/ADI, there is desire to have a
> >>>> generic admin queue not attached to device type.
> >>>>> And AQ in this proposal exactly serves this purpose.
> >>>>>
> >>>>> Device configuring its own IMS vector vs PCI PF configuring VF's MSI-X max
> >>>> vector count are two different functionality.
> >>>>> Both of these commands can ride on a generic queue.
> >>>>> However the queue is not same, because PF owns its own admin queue
> >>>>> (for vf msix config), VF or SF operates its own admin queue (for IMS
> >>>>> config).
> >>>> So I think in the next version we need to clarify:
> >>>>
> >>>> 1) is there a single admin virtqueue shared by all the VFs and PF
> >>>>
> >>>> or
> >>>>
> >>>> 2) per VF/PF admin virtqueue, and how does the driver know how to find the
> >>>> corresponding admin virtqueue
> >>>>
> >>> Admin queue is not per VF.
> >>> Lets take concrete examples.
> >>> 1. So for example, PCI PF can have one AQ.
> >>> This AQ carries command to query/config MSI-X vector of VFs.
> >>>
> >>> 2. In second example, PCI PF is creating/destroying SFs. This is again done by using the AQ of the PCI PF.
> >>>
> >>> 3. A PCI VF has its own AQ to configure some of its own generic attribute, don't know which is that today.
> >>> May be something that is extremely hard to do over features bit.
> >>> Currently proposed v2 doesn't restrict admin queue to be within PCI PF or VF or that matter not limited to other transports.
> >>>
> >>>>> So a good example is,
> >>>>> 1. PCI PF configures 8 MSI-X or 16 IMS vectors for the VF using PF_AQ in HV.
> >>>>> 2. PCI VF when using IMS configures, IMS data, vector, mask etc using VF_AQ
> >>>> in GVM.
> >>>>> Both the functions will have AQ feature bit set.
> >>>> Where did the VF_AQ sit? I guess it belongs to the VF. But if this is
> >>>> true, don't we need some kind of address isolation like PASID?
> >>>>
> >>> Above one for IMS is not a good example. I replied the reasoning last week for it.
> >>>
> >>>>> Fair enough, so we have more users of admin queue than just MSI-X config.
> >>>> Well, what I really meant is that we actually have more users of IMS.
> >>>> That's is exactly what virito-mmio wants. In this case introducing admin
> >>>> queue looks too heavyweight for that.
> >>>>
> >>> IMS config cannot be done over AQ as described in previous email in this thread.
> >>>
> >>>>>>> 2. AQ to follows IN_ORDER and INDIRECT_DESC negotiation like rest of
> >>>>>>> the queues 3. Update commit log to describe why config space is not
> >>>>>>> chosen (scale, on-die registers, uniform way to handle all aq cmds)
> >>>>>> I fail to understand the scale/registeres issues. With the one of my previous
> >>>>>> proposal (device selector), technically we don't even need any config space
> >>>> or
> >>>>>> BAR for VF or SF by multiplexing the registers for PF.
> >>>>>>
> >>>>> Scale issue is: when you want to create, query, manipulate hundreds of
> >>>> objects, having shared MMIO register or configuration register, will be too
> >>>> slow.
> >>>>
> >>>>
> >>>> Ok, this need to be clarified in the commit log. And we need make sure
> >>>> it's not an issue that is only happen for some specific vendor.
> >>> It is present in the v2 commit log cover letter.
> >>> Please let me know if you think it should be in the actual patch commit log.
> >>>
> >>>
> >>>>> And additionally such register set doesn't scale to allow sharing large
> >>>> number of bytes as DMA cannot be done.
> >>>>
> >>>>
> >>>> That's true.
> >>>>
> >>>>
> >>>>>    From physical device perspective, it doesnât scale because device needs to
> >>>> have those resources ready to answer on MMIO reads and for hundreds to
> >>>> thousand of devices it just cannot do it.
> >>>>> This is one of the reason for birth of IMS.
> >>>> IMS allows the table to be stored in the memory and cached by the device
> >>>> to have the best scalability. But I had other questions:
> >>>>
> >>>> 1) if we have a single admin virtqueue, there will still be contention
> >>>> in the driver side
> >>>>
> >>> AQ inherently allows out of order commands execution.
> >>> It shouldn't face contention. For example 1K depth AQ should be serving hundreds of descriptors commands in parallel for SF creation, VF MSI-X config and more.
> >>>
> >>> Which area/commands etc you think can lead to the contention?
> >>>
> >>>> 2) if we have per vf admin virtqueue, it still doesn't scale since it
> >>>> occupies more hardware resources
> >>>>
> >>> That is too heavy, and doesnât scale. Proposal is to not have per vf admin queue.
> >>> Proposal is to have one admin queue in a virtio device.
> >> Right ? where did we mention something that can imply otherwise ?
> > Well, I don't know but probably this part,
> >
> > " PCI VF when using IMS configures, IMS data, vector, mask etc using VF_AQ ..."
> >
> >>
> >>>>>> I do see one advantage is that the admin virtqueue is transport
> >>>> independent
> >>>>>> (or it could be used as a transport).
> >>>>>>
> >>>>> I am yet to read the transport part from [1].
> >>>> Yes, the main goal is to be compatible with SIOV.
> >>>>
> >>> Admin queue is a command interface transport where higher layer services can be buit.
> >>> This includes SR-IOV config, SIOV config.
> >>> And v2 enables SIOV commands implementation whenever they are ready.
> >>>
> >>>>>>> 4. Improve documentation around msix config to link to sriov section of
> >>>> virtio
> >>>>>> spec
> >>>>>>> 5. Describe error that if VF is bound to the device, admin commands
> >>>>>> targeting VF can fail, describe this error code
> >>>>>>> Did I miss anything?
> >>>>>>>
> >>>>>>> Yet to receive your feedback on group, if/why is it needed and, why/if it
> >>>> must
> >>>>>> be in this proposal, what pieces prevents it do as follow-on.
> >>>>>>> Cornelia, Jason,
> >>>>>>> Can you please review current proposal as well before we revise v2?
> >>>>>> If I understand correctly, most of the features (except for the admin
> >>>>>> virtqueue in_order stuffs) are not specific to the admin virtqueue. As
> >>>>>> discussed in the previous versions, I still think it's better:
> >>>>>>
> >>>>>> 1) adding sections in the basic device facility or data structure for
> >>>>>> provisioning and MSI
> >>>>>> 2) introduce admin virtqueue on top as an device interface for those
> >>>>>> features
> >>>>>>
> >>>>> I didn't follow your suggestion. Can you please explain?
> >>>>> Specifically "data structure for provisioning and MSI"..
> >>>> I meant:
> >>>>
> >>>> There's a chapter "Basic Facilities of a Virtio Device", we can
> >>>> introduce the concepts there like:
> >>>>
> >>>> 1) Managed device and Management device (terminology proposed by
> >>>> Michael), and can use PF and VF as a example
> >>>>
> >>>> 2) Managed device provisioning (the data structure to specify the
> >>>> attributes of a managed device (VF))
> >>>>
> >>>> 3) MSI
> >>>>
> >>> Above is good idea. Will revisit v2, if it is not arranged this way.
> >> Let me make sure I understand, you would like to see a new chapter under
> >> "Basic Facilities of a Virtio Device" that is
> >>
> >> called "Device management" and this chapter will explain in few words
> >> the concept
> > Yes.
> >
> >> and it will point to another chapter under "Basic Facilities
> >> of a Virtio Device"
> >>
> >> that was introduced here "Admin Virtqueues" ?
> > So far as I see from the proposal, it needs belong to PCI transport
> > part or a new transport.
>
> No it's not.
>
> It should stay in the basic/generic area like we discussed in the past
> and already agreed on.
>
> Lets move forward please.

Yes, for the general admin virtqueue part, it should be fine, but for
SR-IOV ATTRS part, is it better to move it to PCI transport?

>
> >> So you do agree that managing a managed (create/destroy/setup/etc...)
> >> will be done using the AQ of the managing device ?
> > I agree.
> >
> > Thanks
>
> Ok so I guess we agree on the concept of this patch set and the AQ.

Yes.

Thanks

>
> Thanks.
>
> >
> >>>> And then we can introduced admin virtqueue in either
> >>>>
> >>>> 1) transport part
> >>>>
> >>>> or
> >>>>
> >>>> 2) PCI transport
> >>>>
> >>> It is not specific to PCI transport, and currently it is not a transport either.
> >>> So admin queue will keep as general entity for admin work.
> >>>
> >>>> In the admin virtqueue, there will be commands to provision and
> >>>> configure MSI.
> >>>>
> >>> Please review v2 if it is not arranged this way.
> >>>
> >>>>>> The leaves the chance for future extensions to allow those features to
> >>>>>> be used by transport specific interface which will benefit for
> >>>>>>
> >>>>> AQ allows communication (command, response) between driver and device
> >>>> in transport independent way.
> >>>>> Sometimes it query/set transport specific fields like MSI-X vectors of VF.
> >>>>> Sometimes device configure its on IMS interrupt.
> >>>>> Something else in future.
> >>>>> So it is really a generic request-response queue.
> >>>> I agree, but I think we can't mandate new features to a specific transport.
> >>>>
> >>> Certainly. Admin queue is transport independent.
> >>> PCI MSI-X configuration is PCI transport specific command, so structures are defined it accordingly.
> >>> It is similar to struct virtio_pci_cap, struct virtio_pci_common_cfg etc.
> >>>
> >>> Any other transport will have transport specific interrupt configuration. So it will be defined accordingly whenever that occurs.
> >>> For example, IMS for VF or IMS for SF.
>