On Thu, 2024-05-30 at 14:45 +0200, Igor Mammedov wrote: > Usability of huge VMs (incl. large amount of vCPUs) heavily depends > on used guest OS. What would work for one might not work for other. > For general purpose OS adding IOMMU is typically necessary to make > vCPUs over 254 usable, for Windows adjusting -smp might make a > difference. > > On the other hand, special built guest with tailored QEMU config > might > work without IOMMU just fine (David was the one who patched KVM to > that > effect if I recall correctly). > So, as far as I know, there is no way for any OS with any configuration to bring up more than 255 vCPUs, without a vIOMMU. IIUIC, it's a matter of number of bits available in the I/O APIC IRQ destination register. Like, with only that available, and it being only 8 bit wide, it's just not doable. In fact, even Linux is actually able to at least start... But it shows the "[Firmware Bug]" lines and only has 255 online vCPUs, with no way of bringing up the other ones (and I haven't stress tested a VM running like that, so I don't know if it's also stable). > What I'm saying is that workload specific customization (guest OS) > is not handled at libvirt layer. > FWIW, this seems more hardware than guest OS related to me... > It's typically upper layers which know what guest OS would be > and it's up to them to customize config to make sure that workload > would be able to use VM as intended. > Ok. But then why Libvirt does not let me define a VM with more than 255 vCPUs and no vIOMMU ? I mean, basing on what you're saying, it seems that such should depend (but, if yes, I'm not sure how....) on guest OS too... doesn't it? Regards, -- Dario Faggioli, Ph.D http://about.me/dario.faggioli Virtualization Software Engineer SUSE Labs, SUSE https://www.suse.com/ ------------------------------------------------------------------- <<This happens because _I_ choose it to happen!>> (Raistlin Majere)
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