Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Avi Kivity <avi@redhat.com>]

  On 08/04/2010 10:57 AM, Paolo Bonzini wrote:
> On 08/03/2010 11:34 PM, Anthony Liguori wrote:
>>>
>>> Comparing (from personal experience) the complexity of the Windows
>>> drivers for Xen and virtio shows that it's not a bad idea at all.
>>
>> Not quite sure what you're suggesting, but I could have been clearer.
>> Instead of having virtio-blk where a virtio disk has a 1-1 mapping to a
>> PCI device, we probably should have just done virtio-scsi.
>
> If you did virtio-scsi you might have as well ditched virtio-pci 
> altogether and provide a single PCI device just like Xen does.  Just 
> make your network device also speak SCSI (which is actually in the 
> spec...), and the same for serial devices.
>
> But now your driver that has to implement its own hot-plug/hot-unplug 
> mechanism rather than deferring it to the PCI subsystem of the OS 
> (like Xen), greatly adding to the complication.  In fact, a SCSI 
> controller's firmware has a lot of other communication channels with 
> the driver besides SCSI commands, and all this would be mapped into 
> additional complexity on both the host side and the guest side.  Yet 
> another reminder of Xen.
>
> Despite the shortcomings, I think virtio-pci is the best example of 
> balancing PV-specific aspects (do not make things too complicated) and 
> "real world" aspects (do not invent new buses and the like).

Making virtio-blk a controller doesn't involve much difficulty.  We add 
LUN to all requests, and send a configuration interrupt (which we 
already have) when a LUN is added or removed.  Add some config space for 
discovering available LUNs.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.