Since the introduction of VSMT in 2.11, the spacing of VCPU ids
between cores is controllable through a machine property instead
of being only dictated by the SMT mode of the host:
cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i
Until recently, the machine code would try to change the SMT mode
of the host to be equal to VSMT or exit. This allowed the rest of
the code to assume that kvmppc_smt_threads() == spapr->vsmt is
always true.
Recent commit "8904e5a750 spapr: Adjust default VSMT value for
better migration compatibility" relaxed the rule. If the VSMT
mode cannot be set in KVM for some reasons, but the requested
CPU topology is compatible with the current SMT mode, then we
let the guest run with kvmppc_smt_threads() != spapr->vsmt.
This breaks quite a few places in the code, in particular when
calculating DRC indexes.
This is what happens on a POWER host with subcores-per-core=2 (ie,
supports up to SMT4) when passing the following topology:
-smp threads=4,maxcpus=16 \
-device host-spapr-cpu-core,core-id=4,id=core1 \
-device host-spapr-cpu-core,core-id=8,id=core2
qemu-system-ppc64: warning: Failed to set KVM's VSMT mode to 8 (errno -22)
This is expected since KVM is limited to SMT4, but the guest is started
anyway because this topology can run on SMT4 even with a VSMT8 spacing.
But when we look at the DT, things get nastier:
cpus {
...
ibm,drc-indexes = <0x4 0x10000000 0x10000004 0x10000008 0x1000000c>;
This means that we have the following association:
CPU core device | DRC | VCPU id
-----------------+------------+---------
boot core | 0x10000000 | 0
core1 | 0x10000004 | 4
core2 | 0x10000008 | 8
core3 | 0x1000000c | 12
But since the spacing of VCPU ids is 8, the DRC for core1 points to a
VCPU that doesn't exist, the DRC for core2 points to the first VCPU of
core1 and and so on...
...
PowerPC,POWER8@0 {
...
ibm,my-drc-index = <0x10000000>;
...
};
PowerPC,POWER8@8 {
...
ibm,my-drc-index = <0x10000008>;
...
};
PowerPC,POWER8@10 {
...
No ibm,my-drc-index property for this core since 0x10000010 doesn't
exist in ibm,drc-indexes above.
...
};
};
...
interrupt-controller {
...
ibm,interrupt-server-ranges = <0x0 0x10>;
With a spacing of 8, the highest VCPU id for the given topology should be:
16 * 8 / 4 = 32 and not 16
...
linux,phandle = <0x7e7323b8>;
interrupt-controller;
};
And CPU hot-plug/unplug is broken:
(qemu) device_del core1
pseries-hotplug-cpu: Cannot find CPU (drc index 10000004) to remove
(qemu) device_del core2
cpu 4 (hwid 8) Ready to die...
cpu 5 (hwid 9) Ready to die...
cpu 6 (hwid 10) Ready to die...
cpu 7 (hwid 11) Ready to die...
These are the VCPU ids of core1 actually
(qemu) device_add host-spapr-cpu-core,core-id=12,id=core3
(qemu) device_del core3
pseries-hotplug-cpu: Cannot find CPU (drc index 1000000c) to remove
This patches all the code in hw/ppc/spapr.c to assume the VSMT
spacing when manipulating VCPU ids.
Fixes: 8904e5a750
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
72194664c8