Fedora Rawhide / libguestfs 1.23.14 now enables -cpu host, +kvmclock
Passing '-cpu host' is meant to cause qemu to pass the same CPUID flags from the host up to the appliance. The major benefit of this is that the appliance can use all CPU features available to optimize eg. checksumming, crypto, RAID XOR operations, zero fill. However in practice this can cause strange problems on some hardware: https://bugzilla.redhat.com/show_bug.cgi?id=870071 This manifests itself as particular operations failing randomly, especially if RAID or crypto is involved. If you enable debugging you would see a kernel stack trace similar to the one shown in the bug above (and possibly many other variations). If you see anything like this, please file a bug. Adding +kvmclock should cause the appliance to use kvm-clock instead of the default (probably tsc). This should improve clock stability on heavily loaded machines, and as far as I know has no downside. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW
On Tue, Aug 06, 2013 at 04:38:59PM +0100, Richard W.M. Jones wrote:
Passing '-cpu host' is meant to cause qemu to pass the same CPUID flags from the host up to the appliance. The major benefit of this is that the appliance can use all CPU features available to optimize eg. checksumming, crypto, RAID XOR operations, zero fill.
However in practice this can cause strange problems on some hardware:
https://bugzilla.redhat.com/show_bug.cgi?id=870071
This manifests itself as particular operations failing randomly, especially if RAID or crypto is involved. If you enable debugging you would see a kernel stack trace similar to the one shown in the bug above (and possibly many other variations).
If you see anything like this, please file a bug.
Unfortunately I did see several problems, so I've modified the patch so that -cpu host (or <cpu mode=host-model> for libvirt) is only used when KVM [hardware virt acceleration] is available. It didn't make sense for TCG anyway. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
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Richard W.M. Jones