Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

On Thu, Jun 24, 2021 at 11:35 AM Jason Wang [off-list ref] wrote:

在 2021/6/23 下午1:50, Yongji Xie 写道:

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On Wed, Jun 23, 2021 at 11:31 AM Jason Wang [off-list ref] wrote:

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在 2021/6/22 下午4:14, Yongji Xie 写道:

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On Tue, Jun 22, 2021 at 3:50 PM Jason Wang [off-list ref] wrote:

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在 2021/6/22 下午3:22, Yongji Xie 写道:

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We need fix a way to propagate the error to the userspace.

E.g if we want to stop the deivce, we will delay the status reset until
we get respose from the userspace?

I didn't get how to delay the status reset. And should it be a DoS
that we want to fix if the userspace doesn't give a response forever?

You're right. So let's make set_status() can fail first, then propagate
its failure via VHOST_VDPA_SET_STATUS.

OK. So we only need to propagate the failure in the vhost-vdpa case, right?

I think not, we need to deal with the reset for virtio as well:

E.g in register_virtio_devices(), we have:

          /* We always start by resetting the device, in case a previous
           * driver messed it up.  This also tests that code path a
little. */
        dev->config->reset(dev);

We probably need to make reset can fail and then fail the
register_virtio_device() as well.

OK, looks like virtio_add_status() and virtio_device_ready()[1] should
be also modified if we need to propagate the failure in the
virtio-vdpa case. Or do we only need to care about the reset case?

[1] https://lore.kernel.org/lkml/20210517093428.670-1-xieyongji@bytedance.com/ (local)

My understanding is DRIVER_OK is not something that needs to be validated:

"

DRIVER_OK (4)
Indicates that the driver is set up and ready to drive the device.

"

Since the spec doesn't require to re-read the and check if DRIVER_OK is
set in 3.1.1 Driver Requirements: Device Initialization.

It's more about "telling the device that driver is ready."

But we don have some status bit that requires the synchronization with
the device.

1) FEATURES_OK, spec requires to re-read the status bit to check whether
or it it was set by the device:

"

Re-read device status to ensure the FEATURES_OK bit is still set:
otherwise, the device does not support our subset of features and the
device is unusable.

"

This is useful for some device which can only support a subset of the
features. E.g a device that can only work for packed virtqueue. This
means the current design of set_features won't work, we need either:

1a) relay the set_features request to userspace

or

1b) introduce a mandated_device_features during device creation and
validate the driver features during the set_features(), and don't set
FEATURES_OK if they don't match.


2) Some transports (PCI) requires to re-read the status to ensure the
synchronization.

"

After writing 0 to device_status, the driver MUST wait for a read of
device_status to return 0 before reinitializing the device.

"

So we need to deal with both FEATURES_OK and reset, but probably not
DRIVER_OK.

OK, I see. Thanks for the explanation. One more question is how about
clearing the corresponding status bit in get_status() rather than
making set_status() fail. Since the spec recommends this way for
validation which is done in virtio_dev_remove() and
virtio_finalize_features().

Thanks,
Yongji