Re: [libvirt] [RFC] cpu_map: Remove pconfig from Icelake-Server CPU model

On 30/09/19 18:16, Jiri Denemark wrote: > On Mon, Sep 30, 2019 at 17:16:27 +0200, Paolo Bonzini wrote: >> On 30/09/19 16:31, Hu, Robert wrote: >>>> This might be a problem if there are plans to eventually make KVM support >>>> pconfig, though. Paolo, Robert, are there plans to support pconfig in KVM in the >>>> future? >>> [Robert Hoo] >>> Thanks Eduardo for efforts in resolving this issue, introduced from my Icelake CPU >>> model patch. >>> I've no idea about PCONFIG's detail and plan. Let me sync with Huang, Kai and answer >>> you soon. >> >> It's really, really unlikely. It's possible that some future processor >> overloads PCONFIG in such a way that it will become virtualizable, but >> not IceLake. > > I guess, the likelihood of this happening would be similar to > reintroducing other features, such as osxsave or ospke, right? No, haveing osxsave and ospke was a mistake in the first place (they are not CPU features at all; they are more like a special way to let unprivileged programs read some bits of CR4). For pconfig, it's just very unlikely. >> Would it make sense for libvirt to treat absent CPU flags as "default >> off" during migration, so that it can leave out the flag in the command >> line if it's off? If it's on, libvirt would pass pconfig=on as usual. >> This is a variant of [2], but more generally applicable: >> >>> [2] However starting a domain with Icelake-Server so that it can be >>> migrated or saved/restored on QEMU in 3.1.1 and 4.0.0 would be >>> impossible. This can be solved by a different hack, which would drop >>> pconfig=off from QEMU command line. > > The domain XML does not contain a complete list of all CPU features. > Features which are implicitly included in a CPU model are not listed in > the XML. Count in the differences in libvirt's vs QEMU's definitions of > a particular CPU model and you can see feat=off cannot be mechanically > dropped from the command line as the CPU model itself could turn it on > by default and thus feat=off is not redundant. I think I wasn't very clear, I meant "unsupported by QEMU" when I said "absent". Libvirt on the destination knows that from query-cpu-model-expansion, so it can leave off pconfig if it is not supported by the destination QEMU.