I'm re-reading this, and: On Tue, Aug 24, 2021 at 07:07:58PM +0300, Maxim Levitsky wrote: [...] > Hi, > > Not a classical review but, > I did some digital archaeology with this one, trying to understand what is going on: > > > I think that 16 bit vcpu bitmap is due to the fact that IOAPIC spec states that > it can address up to 16 cpus in physical destination mode. > > In logical destination mode, assuming flat addressing and that logical id = 1 << physical id > which KVM hardcodes, it is also only possible to address 8 CPUs. > > However(!) in flat cluster mode, the logical apic id is split in two. > We have 16 clusters and each have 4 CPUs, so it is possible to address 64 CPUs, > and unlike the logical ID, the KVM does honour cluster ID, > thus one can stick say cluster ID 0 to any vCPU. > > > Let's look at ioapic_write_indirect. > It does: > > -> bitmap_zero(&vcpu_bitmap, 16); > -> kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq, &vcpu_bitmap); > -> kvm_make_scan_ioapic_request_mask(ioapic->kvm, &vcpu_bitmap); // use of the above bitmap > > > When we call kvm_bitmap_or_dest_vcpus, we can already overflow the bitmap, > since we pass all 8 bit of the destination even when it is physical. > > > Lets examine the kvm_bitmap_or_dest_vcpus: > > -> It calls the kvm_apic_map_get_dest_lapic which > > -> for physical destinations, it just sets the bitmap, which can overflow > if we pass it 8 bit destination (which basically includes reserved bits + 4 bit destination). How exactly do you think kvm_apic_map_get_dest_lapic() can overflow? It never writes beyond `bitmap[0]`, as far as I can see. > > > -> For logical apic ID, it seems to truncate the result to 16 bit, which isn't correct as I explained > above, but should not overflow the result. > > > -> If call to kvm_apic_map_get_dest_lapic fails, it goes over all vcpus and tries to match the destination > This can overflow as well. > > [...] -- Eduardo
