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Subject: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in
 Userspace
To:     Yongji Xie <xieyongji@bytedance.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <d2334f66-907c-2e9c-ea4f-f912008e9be8@redhat.com>
Date:   Thu, 24 Jun 2021 11:34:46 +0800
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在 2021/6/23 下午1:50, Yongji Xie 写道:
> On Wed, Jun 23, 2021 at 11:31 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/6/22 下午4:14, Yongji Xie 写道:
>>> On Tue, Jun 22, 2021 at 3:50 PM Jason Wang <jasowang@redhat.com> wrote:
>>>> 在 2021/6/22 下午3:22, Yongji Xie 写道:
>>>>>> 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/


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.

Thanks


>
> Thanks,
> Yongji
>