[libvirt] [PATCH] "pclmuldq" was introduced with Westmere, not Sandy Bridge. This feature is important to get proper performance for aes-128-gcm in openssl, an important cipher for https communication.

Tue Oct 14 11:02:18 UTC 2014

On Tue, Oct 14, 2014 at 01:59:15PM +0300, Itamar Heim wrote:
> On 10/14/2014 01:28 PM, Jan-Frode Myklebust wrote:
> >On Wed, Oct 08, 2014 at 09:49:35AM +0200, Jiri Denemark wrote:
> >>
> >>Technically you are correct and even QEMU added this feature to Westmere
> >>in April 2013. However, our goal is to provide stable virtual hardware
> >>that doesn't change when, e.g., a domain is migrated to another machine
> >>(let's ignore the fact we don't currently enforce such stability for CPU
> >>models/features because of missing functionality in both QEMU and
> >>libvirt). Thus we should not really change existing CPU models. We may
> >>be able to do that in the future depending how (if ever) we solve the
> >>CPU definition probing in QEMU and how libvirt will make use of it to
> >>really enforce stable ABI for guest operating systems.
> >
> >
> >Right, I see the problem, but am having a bit trouble accepting that all
> >our 20 RHEV-H westmere hypervisors are basicly downgraded to nehalem
> >feature-set permanently because if this, and we probably have to live
> >with these servers for quite some time.  If you can't fix existing virtual
> >cpu types, maybe you should add a "westmere-full-feature" cpu type, or
> >similar? And probably also add "rdtscp" which also is missing from the
> >virtual westmere.
> 
> if libvirt would change the definition, then a mixed cluster (old and new
> libvirt) would be broken. it will only work if its a new cpu model

Management apps aren't restricted to using an exact CPU model. They are
free to turn on/off extra features relative to the CPU model's bultin
features.  So using 'rdtscp' doesn't require libvirt to provide a new
CPU model - RHEV can just add <feature name='rdtscp'/> to the XML it
sends to libvirt if it wants to. Of course you need to consider cross
node compat when doing so still.

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|