Arch Linux

Kenni

Member

From: Denmark

Registered: 2007-01-25

Posts: 64

Re: kvm: qemu with option "-enable-kvm" or kvm-84 from AUR?

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and then hand-installs those components without the kernel modules meant to go with it.  Unless the package builder knows something the KVM developer doesn't, I wouldn't go around using that qemu build with whatever KVM module version from whatever different kernel version you may have willy-nilly.

I'm not quite with you on this one. As far as I know, skipping the kernel modules is a fully valid way to install KVM userspace, as long as you have a newer kernel version. If you look at the download page, http://www.linux-kvm.org/page/Downloads, you'll see the following statements:

If you use a kernel from http://www.kernel.org or one provided from your distribution and do not use the modules provided by kvm releases:

    * your kernel has to be 2.6.25 or newer to run the kvm 76 userspace (or any newer release)

There's also a page called "Choosing the right kvm & kernel version", http://www.linux-kvm.org/page/Choose_th … el_version:

KVM come with a linux module to support full virtualization. The linux module is just three files: kvm.ko, kvm_intel.ko, kvm_amd.ko. You can install them just like you install drivers for your video card. The good news is, you might do not need it install it. The 2.6.20 version has already included those linux modules.

If I had the hankering to run the KVM maintained userspace stuff, I'd do what they said and run their kernel stuff too.  I think a proper KVM build should do that.

Well, perhaps, but I don't see any reason for doing this. The KVM developers recommends to stick with the in-kernel KVM modules if you want a stable version and recommends to use the latest KVM-xx modules if you need a new feature or a bug fix (a KVM developer wrote this on the list a few months ago - I can't find the link right now). Do you know of any important features or bug fixes which are present in the KVM-85 kernel modules compared to the current kernel modules included with kernel 2.6.29.2? I haven't had any issues for quite a while with the in-kernel modules.

slinkygn

Member

Registered: 2009-05-03

Posts: 15

Re: kvm: qemu with option "-enable-kvm" or kvm-84 from AUR?

Well, perhaps, but I don't see any reason for doing this. The KVM developers recommends to stick with the in-kernel KVM modules if you want a stable version and recommends to use the latest KVM-xx modules if you need a new feature or a bug fix (a KVM developer wrote this on the list a few months ago - I can't find the link right now). Do you know of any important features or bug fixes which are present in the KVM-85 kernel modules compared to the current kernel modules included with kernel 2.6.29.2? I haven't had any issues for quite a while with the in-kernel modules.

Sure, they say to stick with the in-kernel modules for stable -- combined with qemu, not combined with kvm userspace.  And as far as the kvm-userspace compatibility numbers in that link, yes, I saw that too -- and noticed the last mentioned version (as you note) is 2.6.25.  There have been substantial changes in the in-kernel KVM between .25 and .29.  The fact that the page you quote addresses .20, .21, .22, and .25, but then has no details more recent than .25 and kvm build 76, when we're now on .29 and kvm build 85 -- yeah, I find that enough reason to avoid the combination, when considered together with the fact that the developer recommends sticking with the kernel build of the module and yet has continued to "roll his own" through all that time, and explicitly states to use his kernel with his userspace code.  I think it's more reasonable to think that the stuff about .25 and 76 is an outdated page than to think that code not developed in parallel can maintain perfect synchronicity across 10 new builds.

And, again, in the downloads page immediately *above* what you were quoting:
"If you want to use the kernel module and supporting userspace provided by a kvm release (recommended)"...

Still think there are no differences?  The kvm project would seem to disagree with you.

So whether I can think of any important features/bug fixes in KVM-85 is only equally consequential with whether I can think of any in qemu-0.10.3.  (Hey, I actually typed that correctly this time!)  After all, it's clear the two are not being developed in parallel -- the bug fix from a kvm dev for qemu-kvm mentioned earlier in this thread makes that pretty clear.  I don't know the KVM kernel module changelog well enough to know differences between kernel and KVM's build -- I usually track qemu, not kvm -- but I know that they wouldn't keep rolling their own version across 10 builds when kernel already has it if there weren't differences.  Just like qemu -- none of us disagree that they're maintaining a version of qemu that's distinct from the vanilla qemu distribution.  Why would you think they would be doing anything different for their kvm modules vs. kernel's?  They're basically running the -unstable branch, it sounds like.  (Except potentially without upstream qemu improvements.  Again, the qcow2 corruption bug is a *big* deal, and one I haven't heard a peep about from kvm.)

Last edited by slinkygn (2009-05-04 11:46:45)