On 5/29/24 09:41, Peter Krempa wrote: > On Tue, May 28, 2024 at 16:26:18 -0600, Jim Fehlig via Devel wrote: >> Hi All, >> >> I vaguely recall a discussion about $subject, but can't find it now. Perhaps >> buried in another thread. The topic has been raised internally again, and >> I'd like to gauge the community's interest in automatically adding the >> necessary devices/config when user has specified vcpus > 255. > >> The comparison for prior art is a bit of a stretch, but we e.g. add <audio >> type='spice'/> when spice graphics is configured. > > The thing about 'audio' "device" is that it's purely just backend with > no impact on the VM ABI. In fact 'audio' and 'graphics' IMO should not > have been put under <devices> for that reason. > >> I know libvirt has >> generally tried to avoid policy decisions, but it's not clear to me where we >> stand with cases such as this, where every x86 VM with > 255 vcpus needs a >> similarly configured iommu. > > Adding the IOMMU would change the guest ABI, so libvirt can't auto-add > it, unless a VM with > 255 cpus would not start at all. > > In case of IOMMU the absence of the element means that the user doesn't > want an IOMMU, rather than that it was not configured, so you'd have no > way to express a configuration where > 255 cpus are declared but no > IOMMU was used to start it. Migrating such a config would then break. > Right, that's what I had on mind but didn't write it. Sorry. We have VIR_DOMAIN_DEF_PARSE_ABI_UPDATE flag that is meant for this purpose, isn't it? Michal
