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Subject: Re: [virtio-dev] Memory sharing device

Thanks Roman for the reply. Yes, we need sensors, sound, codecs, etc. as well.

For general string passing, yes, perhaps virtio-vsock can be used. However, I have some concerns about virtio-serial and virtio-vsock (mentioned elsewhere in the thread in rely to Stefan's similar comments) around socket API specialization.

Stepping back to standardization and portability concerns, it is also not necessarily desirable to use general pipes to do what we want, because even though that device exists and is part of the spec already, that results in _de-facto_ non-portability. If we had some kind of spec to enumerate such 'user-defined' devices, at least we can have _de-jure_ non-portability; an enumerated device doesn't work as advertised.

virtio-gpu: we have concerns around its specialization to virgl and de-facto gallium-based protocol, while we tend to favor API forwarding due to its debuggability and flexibility. We may use virtio-gpu in the future if/when it provides that general "send api data" capability.]

In any case, I now have a very rough version of the spec in mind (attached as a patch and as a pdf).

The part of the intro in there that is relevant to the current thread:

Note that virtio-serial/virtio-vsock is not considered because they do not
standardize the set of devices that operate on top of them, but in practice,
are often used for fully general devices. Spec-wise, this is not a great
situation because we would still have potentially non portable device
implementations where there is no standard mechanism to determine whether or
not things are portable. virtio-user provides a device enumeration mechanism
to better control this.

In addition, for performance considerations in applications such as graphics
and media, virtio-serial/virtio-vsock have the overhead of sending actual
traffic through the virtqueue, while an approach based on shared memory can
result in having fewer copies and virtqueue messages. virtio-serial is also
limited in being specialized for console forwarding and having a cap on the
number of clients. virtio-vsock is also not optimal in its choice of sockets
API for transport; shared memory cannot be used, arbitrary strings can be
passed without an designation of the device/driver being run de-facto, and the
guest must have additional machinery to handle socket APIs. In addition, on
the host, sockets are only dependable on Linux, with less predictable behavior
from Windows/macOS regarding Unix sockets. Waiting for socket traffic on the
host also requires a poll() loop, which is suboptimal for latency. With
virtio-user, only the bare set of standard driver calls
(open/close/ioctl/mmap/read) is needed, and RAM is a more universal transport
abstraction. We also explicitly spec out callbacks on host that are triggered
by virtqueue messages, which results in lower latency and makes it easy to
dispatch to a particular device implementation without polling.


On Tue, Feb 12, 2019 at 6:03 AM Michael S. Tsirkin <mst@redhat.com> wrote:
On Tue, Feb 12, 2019 at 02:47:41PM +0100, Cornelia Huck wrote:
> On Tue, 12 Feb 2019 11:25:47 +0000
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > * Roman Kiryanov (rkir@google.com) wrote:
> > > > > Our long term goal is to have as few kernel drivers as possible and to move
> > > > > "drivers" into userspace. If we go with the virtqueues, is there
> > > > > general a purpose
> > > > > device/driver to talk between our host and guest to support custom hardware
> > > > > (with own blobs)?Â
> > > >
> > > > The challenge is to answer the following question:
> > > > how to do this without losing the benefits of standartization?Â
> > >
> > > We looked into UIO and it still requires some kernel driver to tell
> > > where the device is, it also has limitations on sharing a device
> > > between processes. The benefit of standardization could be in avoiding
> > > everybody writing their own UIO drivers for virtual devices.
> > >
> > > Our emulator uses a battery, sound, accelerometer and more. We need to
> > > support all of this. I looked into the spec, "5 Device types", and
> > > seems "battery" is not there. We can invent our own drivers but we see
> > > having one flexible driver is a better idea.Â
> >
> > Can you group these devices together at all in their requirements?
> > For example, battery and accelerometers (to me) sound like low-bandwidth
> > 'sensors' with a set of key,value pairs that update occasionally
> > and a limited (no?) amount of control from the VM->host.
> > A 'virtio-values' device that carried a string list of keys that it
> > supported might make sense and be enough for at least two of your
> > device types.
> Maybe not a 'virtio-values' device -- but a 'virtio-sensors' device
> looks focused enough without being too inflexible. It can easily
> advertise its type (battery, etc.) and therefore avoid the mismatch
> problem that a too loosely defined device would be susceptible to.

Isn't virtio-vsock/vhost-vsock a good fit for this kind of general
string passing? People seem to use it exactly for this.

> > > Yes, I realize that a guest could think it is using the same device as
> > > the host advertised (because strings matched) while it is not. We
> > > control both the host and the guest and we can live with this.
> The problem is that this is not true for the general case if you have a
> standardized device type. It must be possible in theory to switch to an
> alternative implementation of the device or the driver, as long as they
> conform to the spec. I think a more concretely specified device type
> (like the suggested virtio-values or virtio-sensors) is needed for that.
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Attachment: 0001-virtio-user-draft-spec.patch
Description: Binary data

Attachment: virtio-v1.1-wd01.pdf
Description: Adobe PDF document

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