No it does not. [LWN.net]

No it does not.

Posted Mar 6, 2009 7:22 UTC (Fri) by khim (subscriber, #9252)
In reply to: Xen: finishing the job by bluefoxicy
Parent article: Xen: finishing the job

Xen may be faster today but this is not intrinsic advantage.

The story with KVM:
1. Userspace asks kernel to the send the packet.
2. Context switch to kernel.
3. Kernel asks "hardware" to send the packet.
4. "Reverse driver" asks the outr kernel to send the packet.
5. Context switch to outer kernel.
7. Outer kernel talks to real hardware.

The story with Xen:
1. Userspace asks kernel to the send the packet.
2. Context switch to kernel.
3. Kernel asks Xen to send the packet.
4. Conext switch to Xen.
5. Xen asks the outer kernel to send the packet.
6. Context switch to Dom0 kernel.
7. Dom0 kernel talks to real hardware.

Context switches are expensive (equal to hundred simple operations or so) and Xen uses one additional context over KVM. This can easily compensate for simpler interface without "reverse driver". That's why there are push to create drivers directly for Xen - this way it'll be faster then KVM... if KVM will not use paravirtualization. I fail to see why it can not use paravirtualization for all devices except CPU (where it has hardware support and so is fast enough already).

In the end Xen can become fast specialized OS, but so can KVM - and which way is faster? Drepper's words are still relevant: neither Xen nor VMWare have any real advantages which cannot be surmounted by giving KVM more time to catch up, i.e., grant it the same time to develop the features.

And if so then why should we include interim solution? It depends on timeframe: everyone agree btrfs will do everything ext4 does, yet ext4 was included anyway. Because btrfs will not be ready for a few more years. If KVM will need few more years to catch up then may be Dom0 support is worth having in the kernel, but if it's only matter of months - the story will be different...

(Log in to post comments)