On Tue, Oct 01, 2019 at 11:20:42 +0200, Paolo Bonzini wrote: > 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. Oh yeah, we should do this (and I plan to do so), but it won't really help us in this case. Although it could potentially save us some work in case we end up in a similar situation. Jirka -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
