Re: [PATCH v8 10/10] Documentation: Add documentation for VDUSE

From: Stefan Hajnoczi
Date: Mon Jul 05 2021 - 08:50:16 EST

On Mon, Jul 05, 2021 at 11:36:15AM +0800, Jason Wang wrote:
> 在 2021/7/4 下午5:49, Yongji Xie 写道:
> > > > OK, I get you now. Since the VIRTIO specification says "Device
> > > > configuration space is generally used for rarely-changing or
> > > > initialization-time parameters". I assume the VDUSE_DEV_SET_CONFIG
> > > > ioctl should not be called frequently.
> > > The spec uses MUST and other terms to define the precise requirements.
> > > Here the language (especially the word "generally") is weaker and means
> > > there may be exceptions.
> > >
> > > Another type of access that doesn't work with the VDUSE_DEV_SET_CONFIG
> > > approach is reads that have side-effects. For example, imagine a field
> > > containing an error code if the device encounters a problem unrelated to
> > > a specific virtqueue request. Reading from this field resets the error
> > > code to 0, saving the driver an extra configuration space write access
> > > and possibly race conditions. It isn't possible to implement those
> > > semantics suing VDUSE_DEV_SET_CONFIG. It's another corner case, but it
> > > makes me think that the interface does not allow full VIRTIO semantics.
> Note that though you're correct, my understanding is that config space is
> not suitable for this kind of error propagating. And it would be very hard
> to implement such kind of semantic in some transports.  Virtqueue should be
> much better. As Yong Ji quoted, the config space is used for
> "rarely-changing or intialization-time parameters".
> > Agreed. I will use VDUSE_DEV_GET_CONFIG in the next version. And to
> > handle the message failure, I'm going to add a return value to
> > virtio_config_ops.get() and virtio_cread_* API so that the error can
> > be propagated to the virtio device driver. Then the virtio-blk device
> > driver can be modified to handle that.
> >
> > Jason and Stefan, what do you think of this way?

Why does VDUSE_DEV_GET_CONFIG need to support an error return value?

The VIRTIO spec provides no way for the device to report errors from
config space accesses.

The QEMU virtio-pci implementation returns -1 from invalid
virtio_config_read*() and silently discards virtio_config_write*()

VDUSE can take the same approach with

> I'd like to stick to the current assumption thich get_config won't fail.
> That is to say,
> 1) maintain a config in the kernel, make sure the config space read can
> always succeed
> 2) introduce an ioctl for the vduse usersapce to update the config space.
> 3) we can synchronize with the vduse userspace during set_config
> Does this work?

I noticed that caching is also allowed by the vhost-user protocol
messages (QEMU's docs/interop/vhost-user.rst), but the device doesn't
know whether or not caching is in effect. The interface you outlined
above requires caching.

Is there a reason why the host kernel vDPA code needs to cache the
configuration space?

Here are the vhost-user protocol messages:

Virtio device config space

| offset | size | flags | payload |

:offset: a 32-bit offset of virtio device's configuration space

:size: a 32-bit configuration space access size in bytes

:flags: a 32-bit value:
- 0: Vhost master messages used for writeable fields
- 1: Vhost master messages used for live migration

:payload: Size bytes array holding the contents of the virtio
device's configuration space


:id: 24
:equivalent ioctl: N/A
:master payload: virtio device config space
:slave payload: virtio device config space

When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
submitted by the vhost-user master to fetch the contents of the
virtio device configuration space, vhost-user slave's payload size
MUST match master's request, vhost-user slave uses zero length of
payload to indicate an error to vhost-user master. The vhost-user
master may cache the contents to avoid repeated

:id: 25
:equivalent ioctl: N/A
:master payload: virtio device config space
:slave payload: N/A

When ``VHOST_USER_PROTOCOL_F_CONFIG`` is negotiated, this message is
submitted by the vhost-user master when the Guest changes the virtio
device configuration space and also can be used for live migration
on the destination host. The vhost-user slave must check the flags
field, and slaves MUST NOT accept SET_CONFIG for read-only
configuration space fields unless the live migration bit is set.


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