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Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2

From: Paolo Bonzini
Subject: Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?
Date: Wed, 04 Aug 2010 09:57:59 +0200
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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).


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