Re: Enabling hypervisor agnosticism for VirtIO backends

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

Hi Christopher,

On Tue, Sep 07, 2021 at 11:09:34AM -0700, Christopher Clark wrote:
> On Tue, Sep 7, 2021 at 4:55 AM AKASHI Takahiro <takahiro.akashi@linaro.org>
> wrote:
> 
> > Hi,
> >
> > I have not covered all your comments below yet.
> > So just one comment:
> >
> > On Mon, Sep 06, 2021 at 05:57:43PM -0700, Christopher Clark wrote:
> > > On Thu, Sep 2, 2021 at 12:19 AM AKASHI Takahiro <
> > takahiro.akashi@linaro.org>
> > > wrote:
> >
> > (snip)
> >
> > > >    It appears that, on FE side, at least three hypervisor calls (and
> > data
> > > >    copying) need to be invoked at every request, right?
> > > >
> > >
> > > For a write, counting FE sendv ops:
> > > 1: the write data payload is sent via the "Argo ring for writes"
> > > 2: the descriptor is sent via a sync of the available/descriptor ring
> > >   -- is there a third one that I am missing?
> >
> > In the picture, I can see
> > a) Data transmitted by Argo sendv
> > b) Descriptor written after data sendv
> > c) VirtIO ring sync'd to back-end via separate sendv
> >
> > Oops, (b) is not a hypervisor call, is it?
> >
> 
> That's correct, it is not - the blue arrows in the diagram are not
> hypercalls, they are intended to show data movement or action in the flow
> of performing the operation, and (b) is a data write within the guest's
> address space into the descriptor ring.
> 
> 
> 
> > (But I guess that you will have to have yet another call for notification
> > since there is no config register of QueueNotify?)
> >
> 
> Reasoning about hypercalls necessary for data movement:
> 
> VirtIO transport drivers are responsible for instantiating virtqueues
> (setup_vq) and are able to populate the notify function pointer in the
> virtqueue that they supply. The virtio-argo transport driver can provide a
> suitable notify function implementation that will issue the Argo hypercall
> sendv hypercall(s) for sending data from the guest frontend to the backend.
> By issuing the sendv at the time of the queuenotify, rather than as each
> buffer is added to the virtqueue, the cost of the sendv hypercall can be
> amortized over multiple buffer additions to the virtqueue.
> 
> I also understand that there has been some recent work in the Linaro
> Project Stratos on "Fat Virtqueues", where the data to be transmitted is
> included within an expanded virtqueue, which could further reduce the
> number of hypercalls required, since the data can be transmitted inline
> with the descriptors.
> Reference here:
> https://linaro.atlassian.net/wiki/spaces/STR/pages/25626313982/2021-01-21+Project+Stratos+Sync+Meeting+notes
> https://linaro.atlassian.net/browse/STR-25

Ah, yes. Obviously, "fatvirtqueue" has pros and cons.
One of cons is that it won't be suitable for bigger payload
with limited space in descriptors.

> As a result of the above, I think that a single hypercall could be
> sufficient for communicating data for multiple requests, and that a
> two-hypercall-per-request (worst case) upper bound could also be
> established.

When it comes to the payload or data plane, "fatvirtqueue" as well as
Argo utilizes copying. You dub it "DMA operations".
A similar approach can be also seen in virtio-over-ivshmem, where
a limited amount of memory are shared and FE will allocate some space
in this buffer and copy the payload into there. Those allocation will
be done via dma_ops of virtio_ivshmem driver. BE, on the other hand,
fetches the data from the shared memory by using the "offset" described
in a descriptor.
Shared memory is divided into a couple of different groups;
one for read/write for all, others for one writer with many readers.
(I hope I'm right here :)

Looks close to Argo? What is different is who is responsible for
copying data, the kernel or the hypervisor.
(Yeah, I know that Argo has more crucial aspects like access controls.)

In this sense, ivshmem can also be a candidate for hypervisor-agnostic
framework. Jailhouse doesn't say so explicitly AFAIK.
Jan may have some more to say.

Thanks,
-Takahiro Akashi


> Christopher
> 
> 
> 
> >
> > Thanks,
> > -Takahiro Akashi
> >
> >
> > > Christopher
> > >
> > >
> > > >
> > > > Thanks,
> > > > -Takahiro Akashi
> > > >
> > > >
> > > > > * Here are the design documents for building VirtIO-over-Argo, to
> > > > support a
> > > > >   hypervisor-agnostic frontend VirtIO transport driver using Argo.
> > > > >
> > > > > The Development Plan to build VirtIO virtual device support over Argo
> > > > > transport:
> > > > >
> > > >
> > https://openxt.atlassian.net/wiki/spaces/DC/pages/1696169985/VirtIO-Argo+Development+Phase+1
> > > > >
> > > > > A design for using VirtIO over Argo, describing how VirtIO data
> > > > structures
> > > > > and communication is handled over the Argo transport:
> > > > >
> > https://openxt.atlassian.net/wiki/spaces/DC/pages/1348763698/VirtIO+Argo
> > > > >
> > > > > Diagram (from the above document) showing how VirtIO rings are
> > > > synchronized
> > > > > between domains without using shared memory:
> > > > >
> > > >
> > https://openxt.atlassian.net/46e1c93b-2b87-4cb2-951e-abd4377a1194#media-blob-url=true&id=01f7d0e1-7686-4f0b-88e1-457c1d30df40&collection=contentId-1348763698&contextId=1348763698&mimeType=image%2Fpng&name=device-buffer-access-virtio-argo.png&size=243175&width=1106&height=1241
> > > > >
> > > > > Please note that the above design documents show that the existing
> > VirtIO
> > > > > device drivers, and both vring and virtqueue data structures can be
> > > > > preserved
> > > > > while interdomain communication can be performed with no shared
> > memory
> > > > > required
> > > > > for most drivers; (the exceptions where further design is required
> > are
> > > > those
> > > > > such as virtual framebuffer devices where shared memory regions are
> > > > > intentionally
> > > > > added to the communication structure beyond the vrings and
> > virtqueues).
> > > > >
> > > > > An analysis of VirtIO and Argo, informing the design:
> > > > >
> > > >
> > https://openxt.atlassian.net/wiki/spaces/DC/pages/1333428225/Analysis+of+Argo+as+a+transport+medium+for+VirtIO
> > > > >
> > > > > * Argo can be used for a communication path for configuration
> > between the
> > > > > backend
> > > > >   and the toolstack, avoiding the need for a dependency on XenStore,
> > > > which
> > > > > is an
> > > > >   advantage for any hypervisor-agnostic design. It is also amenable
> > to a
> > > > > notification
> > > > >   mechanism that is not based on Xen event channels.
> > > > >
> > > > > * Argo does not use or require shared memory between VMs and
> > provides an
> > > > > alternative
> > > > >   to the use of foreign shared memory mappings. It avoids some of the
> > > > > complexities
> > > > >   involved with using grants (eg. XSA-300).
> > > > >
> > > > > * Argo supports Mandatory Access Control by the hypervisor,
> > satisfying a
> > > > > common
> > > > >   certification requirement.
> > > > >
> > > > > * The Argo headers are BSD-licensed and the Xen hypervisor
> > implementation
> > > > > is GPLv2 but
> > > > >   accessible via the hypercall interface. The licensing should not
> > > > present
> > > > > an obstacle
> > > > >   to adoption of Argo in guest software or implementation by other
> > > > > hypervisors.
> > > > >
> > > > > * Since the interface that Argo presents to a guest VM is similar to
> > > > DMA, a
> > > > > VirtIO-Argo
> > > > >   frontend transport driver should be able to operate with a physical
> > > > > VirtIO-enabled
> > > > >   smart-NIC if the toolstack and an Argo-aware backend provide
> > support.
> > > > >
> > > > > The next Xen Community Call is next week and I would be happy to
> > answer
> > > > > questions
> > > > > about Argo and on this topic. I will also be following this thread.
> > > > >
> > > > > Christopher
> > > > > (Argo maintainer, Xen Community)
> > > > >
> > > > >
> > > >
> > --------------------------------------------------------------------------------
> > > > > [1]
> > > > > An introduction to Argo:
> > > > >
> > > >
> > https://static.sched.com/hosted_files/xensummit19/92/Argo%20and%20HMX%20-%20OpenXT%20-%20Christopher%20Clark%20-%20Xen%20Summit%202019.pdf
> > > > > https://www.youtube.com/watch?v=cnC0Tg3jqJQ
> > > > > Xen Wiki page for Argo:
> > > > >
> > > >
> > https://wiki.xenproject.org/wiki/Argo:_Hypervisor-Mediated_Exchange_(HMX)_for_Xen
> > > > >
> > > > > [2]
> > > > > OpenXT Linux Argo driver and userspace library:
> > > > > https://github.com/openxt/linux-xen-argo
> > > > >
> > > > > Windows V4V at OpenXT wiki:
> > > > > https://openxt.atlassian.net/wiki/spaces/DC/pages/14844007/V4V
> > > > > Windows v4v driver source:
> > > > > https://github.com/OpenXT/xc-windows/tree/master/xenv4v
> > > > >
> > > > > HP/Bromium uXen V4V driver:
> > > > > https://github.com/uxen-virt/uxen/tree/ascara/windows/uxenv4vlib
> > > > >
> > > > > [3]
> > > > > v2 of the Argo test unikernel for XTF:
> > > > >
> > > >
> > https://lists.xenproject.org/archives/html/xen-devel/2021-01/msg02234.html
> > > > >
> > > > > [4]
> > > > > Argo HMX Transport for VirtIO meeting minutes:
> > > > >
> > > >
> > https://lists.xenproject.org/archives/html/xen-devel/2021-02/msg01422.html
> > > > >
> > > > > VirtIO-Argo Development wiki page:
> > > > >
> > > >
> > https://openxt.atlassian.net/wiki/spaces/DC/pages/1696169985/VirtIO-Argo+Development+Phase+1
> > > > >
> > > >
> > > >
> >