Re: [libvirt] [Qemu-devel] Modern CPU models cannot be used with libvirt

On Sat, Mar 10, 2012 at 12:58:43PM -0300, Eduardo Habkost wrote: > On Sat, Mar 10, 2012 at 12:42:46PM +0000, Daniel P. Berrange wrote: >>> >>> I could have sworn we had this discussion a year ago or so, and had decided >>> that the default CPU models would be in something like /usr/share/qemu/cpu-x86_64.conf >>> and loaded regardless of the -nodefconfig setting. /etc/qemu/target-x86_64.conf >>> would be solely for end user configuration changes, not for QEMU builtin >>> defaults. >>> >>> But looking at the code in QEMU, it doesn't seem we ever implemented this ? >> >> Arrrgggh. It seems this was implemented as a patch in RHEL-6 qemu RPMs but, >> contrary to our normal RHEL development practice, it was not based on >> a cherry-pick of an upstream patch :-( >> >> For sake of reference, I'm attaching the two patches from the RHEL6 source >> RPM that do what I'm describing >> >> NB, I'm not neccessarily advocating these patches for upstream. I still >> maintain that libvirt should write out a config file containing the >> exact CPU model description it desires and specify that with -readconfig. >> The end result would be identical from QEMU's POV and it would avoid >> playing games with QEMU's config loading code. > > I agree that libvirt should just write the config somewhere. The problem > here is to define: 1) what information should be mandatory on that > config data; 2) who should be responsible to test and maintain sane > defaults (and where should they be maintained). > > The current cpudef definitions are simply too low-level to require it to > be written from scratch. Lots of testing have to be done to make sure we > have working combinations of CPUID bits defined, so they can be used as > defaults or templates. Not facilitating reuse of those tested > defauls/templates by libvirt is duplication of efforts. > > Really, if we expect libvirt to define all the CPU bits from scratch on > a config file, we could as well just expect libvirt to open /dev/kvm > itself and call the all CPUID setup ioctl()s itself. That's how > low-level some of the cpudef bits are. > s/some/all If libvirt assumes anything about what kvm actually supports it is working only by sheer luck.

> (Also, there are additional low-level bits that really have to be > maintained somewhere, just to have sane defaults. Currently many CPUID > leafs are exposed to the guest without letting the user control them, > and worse: without keeping stability of guest-visible bits when > upgrading Qemu or the host kernel. And that's what machine-types are > for: to have sane defaults to be used as base.) > > Let me give you a practical example: I had a bug report about improper > CPU topology information[1]. After investigating it, I have found out > that the "level" cpudef field is too low; CPU core topology information > is provided on CPUID leaf 4, and most of the Intel CPU models on Qemu > have level=2 today (I don't know why). So, Qemu is responsible for > exposing CPU topology information set using '-smp' to the guest OS, but > libvirt would have to be responsible for choosing a proper "level" value > that makes that information visible to the guest. We can _allow_ libvirt > to fiddle with these low-level bits, of course, but requiring every > management layer to build this low-level information from scratch is > just a recipe to waste developer time. And QEMU become even less usable from a command line. One more point to kvm-tool I guess.