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

On Tue, Jul 06, 2021 at 10:34:33AM +0800, Jason Wang wrote:

在 2021/7/5 下午8:49, Stefan Hajnoczi 写道:

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On Mon, Jul 05, 2021 at 11:36:15AM +0800, Jason Wang wrote:

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在 2021/7/4 下午5:49, Yongji Xie 写道:

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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".

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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*()
accesses.

VDUSE can take the same approach with
VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.

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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?

Because:

1) Kernel can not wait forever in get_config(), this is the major difference
with vhost-user.

virtio_cread() can sleep:

  #define virtio_cread(vdev, structname, member, ptr)                     \
          do {                                                            \
                  typeof(((structname*)0)->member) virtio_cread_v;        \
                                                                          \
                  might_sleep();                                          \
                  ^^^^^^^^^^^^^^

Which code path cannot sleep?

2) Stick to the current assumption that virtio_cread() should always
succeed.

That can be done by reading -1 (like QEMU does) when the read fails.

Stefan