2013-12-16 14:16+0200, Gleb Natapov: > On Mon, Dec 16, 2013 at 01:01:10PM +0100, Radim Krčmář wrote: > > > > - Where does the 'only one supported cluster' come from? > > > > > > > "only one supported cluster" comes from 8 bit cpuid limitation of KVM's > > > x2apic > > > implementation. With 8 bit cpuid you can only address cluster 0 in > > > logical mode. > > > > One x2apic cluster has 16 cpus and we generate the x2apic LDR correctly, > > so 8 bit cpuid can address first 16 clusters as well. > > > > u32 ldr = ((id >> 4) << 16) | (1 << (id & 0xf)); > > > Interrupt from a device cannot generate such ldr, only IPI can. Only > 4 cpus in cluster zero are addressable in clustering mode by a > device. Without irq remapping x2apic is a PV interface between host > and guest where guest needs to know KVM implementation's limitation to > use it.
Thanks, I'll read more about devices ... still no idea how could they address cluster > 15. > I do not see a point in fixing problems in x2apic logical mode > emulation right now since it will not make it usable, as long as > there is not security problems there. Agreed; I wanted to know why this patch was correct, if we cared. > > > > I only see we use 'struct kvm_lapic *logical_map[16][16];', which > > > > supports 16 clusters of 16 apics = first 256 vcpus, so if we map > > > > everything to logical_map[0][0:15], we would not work correctly in > > > > the cluster x2apic, with > 16 vcpus. > > > > > > > Such config cannot work today because of 8 bit cpuid limitation. When the > > > limitation > > > will be removed KMV_X2APIC_CID_BITS will be set to actual number of bits > > > we want to support. > > > > Even with KMV_X2APIC_CID_BITS = 4, which would allow us to support 8 bit > > cpuid, we would still deliver interrupts destined for cpuid > 256 to > > potentially plugged cpus. > Again, KMV_X2APIC_CID_BITS = 4 will not allow us to support 8 bit cpuids > unfortunately, not sure what you mean by the second part of the sentence. Sorry, I meant that with this change, we map all clusters to cluster 0, which has two flaws: - in kvm_lapic_set_base(), the order of vcpu creation determines those assigned to cluster 0, and the rest is unaddressable (overwritten) - we can send IPI to an unplugged high cpuid and it arives in cluster 0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
