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

[Yongji Xie]

On Mon, Jul 5, 2021 at 8:50 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
>
> 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?
>

We add a timeout and return error in case userspace never replies to
the message.

> 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*()
> accesses.
>
> VDUSE can take the same approach with
> VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.
>

I noticed that virtio_config_read*() only returns -1 when we access a
invalid field. But in the VDUSE case, VDUSE_DEV_GET_CONFIG might fail
when we access a valid field. Not sure if it's ok to silently ignore
this kind of error.

Thanks,
Yongji