Libguestfs - Aug 2013 - 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.

-- 
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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
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