Re: [Engine-devel] host cpu feature

Quoting Doron Fediuck <dfediuck(a)redhat.com&gt;: > ----- Original Message ----- >> From: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt; >> To: "Doron Fediuck" <dfediuck(a)redhat.com&gt; >> Cc: "engine-devel" <engine-devel(a)ovirt.org&gt; >> Sent: Wednesday, December 5, 2012 7:14:46 PM >> Subject: Re: [Engine-devel] host cpu feature >> >> >> >> ----- Original Message ----- >> > From: "Doron Fediuck" <dfediuck(a)redhat.com&gt; >> > To: "Yaniv Kaul" <ykaul(a)redhat.com&gt; >> > Cc: "Laszlo Hornyak" <lhornyak(a)redhat.com&gt;, "engine-devel" >> > <engine-devel(a)ovirt.org&gt; >> > Sent: Wednesday, December 5, 2012 6:10:55 PM >> > Subject: Re: [Engine-devel] host cpu feature >> > >> > > Alternative idea, inspired by "Thus, if you hit any bugs, you are >> > > on >> > > your own" (http://libvirt.org/formatdomain.html#elementsCPU wrt >> > > 'host-passthrough'): >> > > A config option to determine if we use host-model or >> > > host-passthrough. >> > > Y. >> > > >> > >> > I do not think the engine should go to this level. >> > ie- it can ask for passthrough as a feature, and the >> > actual implementation is handled by vdsm. >> > >> >> If vdsm decides over host-passthrough or host-model, then how will >> the engine user know what exactly his VM gets. I think vdsm must be >> told exactly what to do. >> > > VDSM maintains some level of independence. This is why it the engine > should be able to ask for passthrough as a feature. Otherwize vdsm will > handle it. So for now I suggest we stick to passthrough only, and if > we get an RFE for advanced mode we'll support the host model. What are we gaining by using passthrough over host-model? Looking at libvirt documentation, it seems that both modes give host CPU capabilities to guest VM. Whereas the downside of passthrough is that it limits migration. Whereas host-model will migrate to other hardware and if the destination hardware is better than source then the guest VM performance can be improved by rebooting guest. As a stretch goal, ovirt can keep track of host capabilities and inform the user after migrating to a better host, that a reboot may improve guest performance.