Not logged in
Log in now
Create an account
Subscribe to LWN
LWN.net Weekly Edition for December 5, 2013
Deadline scheduling: coming soon?
LWN.net Weekly Edition for November 27, 2013
ACPI for ARM?
LWN.net Weekly Edition for November 21, 2013
Let's step back a bit
Posted Jun 3, 2009 19:21 UTC (Wed) by BrucePerens (guest, #2510)
So, the important part of Xen, in that it provides something that KVM doesn't have, is already in the kernel. KVM has a hypervisior already in the kernel. The Xen hypervisor is inelegant.
So, is it possible to make the KVM hypervisor support Dom0?
Posted Jun 3, 2009 20:09 UTC (Wed) by nevets (subscriber, #11875)
KVM developers have no interest (nor have they designed KVM) to work with paravirtualization (the thing needed by the OS to support non virtualization supported hardware). Although, I do believe KVM can make use of virtio, but that's another story.
We have enough in the kernel to support a DomU. That is, a true guest.
But Dom0 is a special guest with Xen. The Xen hypervisor passes off the work of drivers to Dom0 to have it do the work. But this interface between Dom0 and the hypervisor is a bit more intrusive than the interface needed by DomU (and already exists).
The issue is that once we add this Dom0 interface, we will forever need to support it. Because any changes we make will break Xen. This is why I suggested having Linux host the Xen source code. Then we can freely change the Dom0<->hypervisor interface without worrying about breaking an external ABI.
Note, my suggestion is not about Xen being inside Linux. It would still be a micro kernel loaded first. But the vmlinuz image would be one. First we load the Xen hypervisor, and then we load Dom0. This will couple the two tightly and the user would not need to worry about incompatibilities.
Posted Jun 4, 2009 8:42 UTC (Thu) by rwmj (subscriber, #5474)
Bruce, this is an interesting and valid point, but it's also a bit like the discussion of 3D rendering that happened in the mid 90s. Sure, 3D graphics cards were rare and expensive at first, and that meant there was a place for software rendering.
Nowadays though no serious 3D program (ie. no game!) comes with a software renderer, because the 3D hardware is everywhere, on motherboards, in open handhelds like the GP2x-Wiz, and even in experimental boards like the ARM-based Beagleboard.
Hardware virt support is in just about every new x86-64 processor that comes out. A few 32 bit netbooks don't have it right now, but it'll come to those too.
Also don't overlook the fact that KVM does have software
emulation. OK, it's slow, it's in userspace, and it relies on qemu. Nevertheless, just running qemu-kvm will transparently fall back to software
emulation if the hardware doesn't support virtualization.
Posted Jun 4, 2009 8:43 UTC (Thu) by rwmj (subscriber, #5474)
Posted Jun 4, 2009 12:18 UTC (Thu) by nye (guest, #51576)
Posted Jun 4, 2009 17:44 UTC (Thu) by buchanmilne (guest, #42315)
But, an 18-month-old 16-core (8*"Dual Core AMD Opteron(tm) Processor 885") server (Sun X4600-M1) doesn't have it. With another 5 years of lifetime on these boxes, it really would be nice to keep Xen (which is what they are currently running). There's no way I would migrate this (with heavily utilised VMs) to qemu-kvm ...
Posted Jun 4, 2009 21:28 UTC (Thu) by jimparis (subscriber, #38647)
Posted Jun 3, 2009 20:10 UTC (Wed) by dtlin (✭ supporter ✭, #36537)
utility which is able to run xen paravirtualized kernels as guests on linux
hosts, without the xen hypervisor, using kvm instead.
I haven't tried it out, but running Xen DomU on KVM seems perfectly
possible. In any case, KVM and Xen+HVM are about equal in terms of
KVM's "Dom0" is the unmodified Linux kernel, running on bare hardware
— there's nothing special about it. I'm not sure why you'd even
want Xen's Dom0 there?
HVM No HVM
KVM Supports many guests Not possible
Xen Supports manu guests Supports paravirtualized guests
The "not possible" (unless you're satisfied with QEMU) is what the Xen
supporters are really focusing on.
No, it's completely unrelated.
Posted Jun 3, 2009 20:35 UTC (Wed) by gwolf (subscriber, #14632)
KVM is great, say, if you want to run Windows instances - None of them will know (well, except for the hardware self-description strings) they are running virtualized. Same thing, yes, can be specified to Xen.
However, Xen's paravirtualization funcionality is completely unmatched by KVM - Xen can run DomU (guest) kernels that are explicitly aware they are running under a paravirtualized environment. This, of course, excludes non-free software, as they would have to be ported to the Xen pseudo-architecture. However, it is a very popular way to run completely independent Linux systems.
Why do you want to paravirtualize? Because the performance impact is way lower. You don't have to emulate hardware at all - In a regular virtualization setup, the guest OS will still shuffle bits around to give them to, say, the ATA I/O interface, possibly aligning them to cylinder/head/sector - On a hard disk that just does not exist, that is a file on another filesystem or whatever. When it is paravirtualized, the guest OS just signals the host OS to do its magic.
My favorite way out for most of the cases I would be forced to handle with Xen for this kind of needs is to use vserver - Which is _not_ formally a virtualization technology, but a compartmentalization/isolation technology (akin to what was introduced as the BSD Jails around 2000), where many almost-independent hosts share a single kernel, but live within different security contexts.
Posted Jun 4, 2009 1:59 UTC (Thu) by drag (subscriber, #31333)
Well things like BSD Jails, Vserver, OpenVZ, etc etc. All of these are very much virtualization technology in a very real sense. They just are not hardware virtualization.
> Why do you want to paravirtualize? Because the performance impact is way lower. You don't have to emulate hardware at all - In a regular virtualization setup, the guest OS will still shuffle bits around to give them to, say, the ATA I/O interface, possibly aligning them to cylinder/head/sector - On a hard disk that just does not exist, that is a file on another filesystem or whatever. When it is paravirtualized, the guest OS just signals the host OS to do its magic.
Heh. KVM has paravirt drivers that are built into the kernel right now.
virtio-blk = block driver
virtio-rng = random number generator
virtio-net = ethernet network driver
virtio-balloon = used for reclaiming memory from VMs
virtio-pci = pci driver
9pnet_virtio = plan9 networking
And that works fine with updated versions of Qemu also. So you should be able to take advantage of them if your using Kqemu + Qemu for your virtualization. I think. But virtio is a standardized way of doing things. Should probably work with Qemu-dm for Xen stuff.
I there are windows drivers for virtio network. I am not sure about virtio block or balloon though...
I don't know how well KVM + Virtio compares to Xen PV..
Then on top of that you can use AMD's IOMMU or Intel's VT-d to map real hardware directly to virtualized hosts, which would be the fastest possible since your handing off direct access to the hardware.
Posted Jun 4, 2009 7:03 UTC (Thu) by sf_alpha (guest, #40328)
Posted Jun 4, 2009 12:20 UTC (Thu) by drag (subscriber, #31333)
You need to have Intel or AMD's virtualization support to take advantage of KVM.
Even with the virtualization support KVM will be slower then PV. Xen's PV is very superior in terms of performance in almost all situations.
KVM's advantages over Xen are:
* Cleaner design. I am guessing that KVM hypervisor code is between 20k-30k with all the arch it supports were Xen's hypervisor code is easily 10x that much.
* Much easier to administrate and deal with. Does not require patches, does not require rebooting or anything of that nature. It's "just there". Does not require special console software or management tools beyond just qemu if that is all you want. You can use top to monitor VMs and crtl-z to pause them if you started them from a terminal, for example.
* Does not require to have your OS "lifted" into a Dom0... The way Linux interacts with the hardware does not change. This means (with latest kernels) I can suspend my laptop while running VMs and it just works.
* Heavily leverage's Linux's existing features. Instead of having to write various peices of hardware support into the hypervisor KVM gets all that and more by default. When Linux does improvements to, say, memory management then people using KVM directly benefit from that work.
(this is not a huge advantage over Xen, its more of a big improvement when compared to Vmware ESX.. no restrictions to hardware, network block protocols, or sata or anything like that... if Linux supports it you can use it in KVM)
* It is already installed and setup on your machine. All you have to do is intall the qmeu portion and the virt-manager or libvirt stuff if you want to have a nice and easy way to manage them. All Linux distributions have KVM support.. it's modules are by default by everything I've looked at.
* PV on Xen is still easily performance king.
* require some hardware support.
Posted Jun 4, 2009 7:06 UTC (Thu) by bronson (subscriber, #4806)
Wow, people are still writing 9p code? Given the sad state of http://sourceforge.net/projects/v9fs and http://sourceforge.net/projects/npfs I thought that these projects were stone dead.
I'd really like a network filesystem that is easier to administer than NFS and CIFS... Tried DRBD but didn't like it much. Is v9fs worth a look?
Posted Jun 4, 2009 12:03 UTC (Thu) by drag (subscriber, #31333)
But DRBD is a way of keeping volumes in sync, not so much a file system.
The easiest FS to administer that I know of is sshfs. I use it heavily and it is stable and actually very fast. It can beat NFS even sometimes.. And all you need is Openssh server running and a fuse support in the client. The ssh server is the real gauge on how well sshfs works. Anything other then a relatively recent version of OpenSSH and I doubt the results will be that good.
But if DRBD was even being considured then your needs are going to be specialized. Other alternative to look at could possibly be Redhat's GNBD from GFS or ISCSI.
Posted Jun 4, 2009 19:32 UTC (Thu) by bronson (subscriber, #4806)
I only mentioned DRBD to illustrate how desperate I've become! It was actually pretty good except that I couldn't get the split brain recovery to work the way I wanted. So close and yet so far. Haven't gotten desperate enough to try AFS yet!
Why doesn't 9p or webdav or some simple protocol take off? It's amazing to me that NFS and CIFS are still state of the art. I guess I don't understand the trade-offs very well.
Posted Jun 4, 2009 20:20 UTC (Thu) by drag (subscriber, #31333)
For sshfs if you want to have good performance you need to disable compression. If you think the crypto has to much overhead then change the encryption method to RC4.
Very likely you were running something like 3DES that has very high overhead. And like I said you need to have a relatively recent version of OpenSSH (say a version from the past 2 years or so) for reliable service.
You can set these on a per server basis in your ~/.ssh/config
I have had no problem personally beating NFS when it comes to my personal usage at home over wireless and gigabit link.. although of course this sort of thing is not suitable for large numbers of users.
Posted Jun 4, 2009 14:08 UTC (Thu) by sbergman27 (guest, #10767)
I have an ugly situation where I have a (proprietary) cobol C/ISAM <-> SQL gateway to some cobol accounting files. Due to the brain-deadness of the proprietary vendor (political concerns, their licenscing with their Cobol runtime supplier, yadda, yadda, yadda...), I have to run it virtualized in an old distro and it sees the C/ISAM files via NFS4. It's written to do a lot of fsync'ing and doesn't seem to make any use of any sort of NFS caching, and so latency absolutely kills its performance. I can't use any of the virtio stuff because the guest kernel is too old to support it, and even that has latencies in the hundreds of microseconds. So I'm using the software emulated E1000 driver, which is almost as efficient as virtio.
However, if I could use the 9p shared volume stuff, I suspect, but am not sure, that latency would be much improved. As it stands, it is still over twice as fast as running on a separate machine via NFS4 over 1000baseT.
So far as I know, the 9p-virtio thing is still an active project, but not yet in mainline KVM. Or, at least, it does not seem to be in Ubuntu 9.04 server.
Posted Jun 4, 2009 12:48 UTC (Thu) by gwolf (subscriber, #14632)
Yes, and that's good - I use KVM with paravirt network and disk devices for Windows hosts. Still, many things (i.e. memory access, real CPU mapping, even the kind of architecture the guests report as having) have to be emulated. Paravirt devices are a great boost, though - And by being much simpler, say, than hardware-specific drivers, I am also reducing the most common cause for Windows' instability.
Now, both with Xen and with KVM (and I'd expect with any other virtualization technology) you can forward a real device - Just remove support for it on the host (or Dom0) kernel and ask the virtualizer to forward the needed interrupts/mapped memory space/bus address, and you have it natively inside. Of course, you lose the ability to perform live migrations - But you cannot always win! :)
Posted Jun 10, 2009 17:37 UTC (Wed) by tmassey (guest, #52228)
Where would I get paravirt Windows drivers for KVM?
Posted Jun 3, 2009 20:13 UTC (Wed) by ncm (subscriber, #165)
Posted Jun 4, 2009 12:56 UTC (Thu) by gwolf (subscriber, #14632)
When I bought my laptop, January 2008, I shopped explicitly for one with virtualization capability. However, for a long time I just was not able to use it as such - Because of the lack of support in Xen for core features I want a laptop to support, such as ACPI (which is mainly useful for laptops, granted, but that could be very well used everywhere, leading to sensible power savings). Virtualization does not only work at the server farm, it can also be very useful at desktops.
Posted Jun 4, 2009 15:42 UTC (Thu) by TomMD (guest, #56998)
YES! And its not just for x86 anymore, but there are architectures that don't have VT or SVM hackery and are perfectly viable users of Xen. I'd love to run Xen on the (ARM based) beagle board or a BB based laptop.
Posted Jun 4, 2009 20:29 UTC (Thu) by drag (subscriber, #31333)
KVM works fine on other architectures (like PowerPC), so that is all a bit of a red herring.
For x86 systems that donnot have VT/SVM you can use Kqemu and get similar functionality and speed.
Posted Jun 9, 2009 2:11 UTC (Tue) by xyzzy (subscriber, #1984)
Posted Jun 9, 2009 7:50 UTC (Tue) by paulj (subscriber, #341)
Posted Jun 7, 2009 10:41 UTC (Sun) by djao (subscriber, #4263)
When I bought my laptop, January 2008, I shopped explicitly for one with virtualization capability. However, for a long time I just was not able to use it as such - Because of the lack of support in Xen for core features I want a laptop to support, such as ACPI
I bought my laptop in April 2008 and I've been using it with KVM almost from day one. Everything works great, including ACPI.
Posted Jun 4, 2009 13:28 UTC (Thu) by ESRI (guest, #52806)
Posted Jun 3, 2009 22:21 UTC (Wed) by nix (subscriber, #2304)
If you don't have VT support, my understanding is that Xen similarly
works, just slower.
So what's the substantive difference?
Posted Jun 3, 2009 23:01 UTC (Wed) by nevets (subscriber, #11875)
I also think the issue is that Xen is still quite a head of KVM in features, but this too is slowing down.
Posted Jun 4, 2009 2:34 UTC (Thu) by drag (subscriber, #31333)
YES PV is massively faster then just plain Qemu. Massively faster in all respects. The overhead of Xen PV vs naked hardware is going to be just a few percent.
Of course this requires modification to the guest.
Copyright © 2013, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds