Re: [RFC 1/4] New virtio bus driver

[Avi Kivity <avi@qumranet.com>]

Arnd Bergmann wrote:
> On Monday 09 July 2007, Avi Kivity wrote:
>   
>>> That is much harder to do, it would require a separate interrupt if you
>>> want to have a device independent mechanism. Most real buses (like PCI)
>>> don't have this, so I'm not sure it's good to do this at the virtio
>>> layer. You already listed the media change notification, which can
>>> be handled without this by either removing and adding the complete
>>> virtio device, or by using a channel for out-of-band data like scsi.
>>>       
>> At the very least, we need an interrupt for device hotplug.  That same
>> interrupt can be used to notify the guest about configuration changes
>> within a device ("rescan the virtbus and all who sail in her").
>>     
>
> There is an important difference between device reconfiguration and bus
> reconfiguration (device hotplug). If we want device reconfiguration, that
> needs to be implemented within the virtio layer, because the device
> drivers need to know about it. For hotplugging devices, we need to have
> a bus specific mechanism, while the device drivers only care about
> 'new device found' and 'device is gone' events that are already supported
> by any linux bus_type.
>   

Sure.  I'm only suggesting to share the interrupt.

> The problem with hotplugging devices on PCI is that there is not a good
> standard for this yet. The best option we have is probably PCI express
> hotplug, but I'm not sure how many legacy operating systems already support
> this.
>   

This is indeed a problem.  Perhaps we should have a virtqueue for 
reconfiguration (with a couple of read messages always queued for 
host-side changes, and a write message sent when the guest reconfigures).

-- 
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