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

[Yongji Xie <xieyongji@bytedance.com>]

On Tue, Jul 6, 2021 at 6:22 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Tue, Jul 06, 2021 at 11:04:18AM +0800, Yongji Xie wrote:
> > On Mon, Jul 5, 2021 at 8:50 PM Stefan Hajnoczi <stefanha@redhat.com> 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.
>
> That's a good point but it's a general VIRTIO issue. Any device
> implementation (QEMU userspace, hardware vDPA, etc) can fail, so the
> VIRTIO specification needs to provide a way for the driver to detect
> this.
>
> If userspace violates the contract then VDUSE needs to mark the device
> broken. QEMU's device emulation does something similar with the
> vdev->broken flag.
>
> The VIRTIO Device Status field DEVICE_NEEDS_RESET bit can be set by
> vDPA/VDUSE to indicate that the device is not operational and must be
> reset.
>

It might be a solution. But DEVICE_NEEDS_RESET  is not implemented
currently. So I'm thinking whether it's ok to add a check of
DEVICE_NEEDS_RESET status bit in probe function of virtio device
driver (e.g. virtio-blk driver). Then VDUSE can make use of it to fail
device initailization when configuration space access failed.

Thanks,
Yongji