On Fri, 2024-05-31 at 11:16 +0100, Joao Martins wrote: > On 30/05/2024 18:00, Dario Faggioli via Devel wrote: > > > > 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. > > > > On VMs, there's alternatively a KVM PV op to bump the limit to 32k > vCPUs i.e. > > KVM_FEATURE_MSI_EXT_DEST_ID > (on qemu it's cpu feature name +kvm-msi-ext-dest-id) > > Which uses the other 24-bits for that destination register (which on > hardware > would cross a page boundary in IOAPIC entry IIUC) without needing > IOMMU > interrupt remapping. But you need the guest to understand that > feature (which is > there since Linux v5.15 or around that timeframe). I think this is > what Igor is > referring to. > Ok, and thanks for the explanation. :-) Now, it may very well be me, but this confuses me even more... :-O So, right now, if you try to create a VM with more than 255 vCPUs and do not explicitly and manually add a vIOMMU to it, Libvirt does not let you do that. In fact, as show in earlier messages, both `virsh edit` and `virsh define` blocks you until you either reduce the vCPUs number or add the device. In fact, if I want a VM with 256 vCPUs and no vIOMMU, I just can't have it. And I don't think that check is guest-OS (or guest-OS-kernel- version) dependent so, even if the guest is a Linux with a > 5.15 kernel that understands that feature, and hence things could actually work there, we force users to defin a vIOMMU. I guess that what I do not understand is the coexistence of those checks and the decision of not adding the device automatically (and even less now that you told me that it's not even always strictly necessary). Basically, we don't want to create a vIOMMU automatically, because it might be that things work without a vIOMMU, and users may not want the vIOMMU. But if there's no vIOMMU in the xml, we don't even define the VM and ask the users to go and put there a vIOMMU themselves? What am I missing? Thanks and 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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