XICS needs to know the highest VCPU id that may be presented to the
guest plus 1. Commit f303f117fec3 "spapr: ensure we have at least one
XICS server" changed how the maximum is computed from:
smp_cpus * kvmppc_smt_threads() / smp_threads
to:
DIV_ROUND_UP(smp_cpus * kvmppc_smt_threads(), smp_threads)
This was done because at the time we could pass broken CPU topologies
to the -smp command line options, such as threads=9,cpus=1. On a POWER8
host this would give:
1 * 8 / 9 == 0 servers
and cause QEMU to crash later during XICS setup.
The formulat evolved a bit to accomodate CPU hot-plug and VSMT, but
most important, stricter checks are performed on the CPU topology.
With -smp threads=9,cpus=1:
qemu-system-ppc64:
cpu topology: sockets (1) * cores (1) * threads (9) > maxcpus (1)
With -smp threads=9,maxcpus=1:
qemu-system-ppc64: maxcpus must be equal to or greater than smp
More generally, machine types with hotplug support (2.7 and up), no
longer allow to set maxcpus or smp_cpus to a value that isnt't a
multiple of smp_threads.
With -smp threads=4,cpus=6:
qemu-system-ppc64: smp_cpus (6) must be multiple of threads (4)
With -smp threads=4,maxcpus=6:
qemu-system-ppc64: max_cpus (6) must be multiple of threads (4)
This means that the division is perfect and we don't need DIV_ROUND_UP(),
and we could do a regular division:
max_cpus * spapr->vsmt / smp_threads
So this patch changes xics_max_server_number() to use the spapr_vcpu_id(),
which works too since max_cpus is a multiple of smp_threads:
(max_cpus / smp_threads ) * spapr->vsmt + max_cpus % smp_threads
It breaks migration of pre-2.7 machine types with unusual CPU topologies,
but I guess this is an acceptable trade-off.
Signed-off-by: Greg Kurz <gr...@kaod.org>
---
hw/ppc/spapr.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 800d3f001253..f1722214cc74 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -176,7 +176,7 @@ static void pre_2_10_vmstate_unregister_dummy_icp(int i)
static int xics_max_server_number(sPAPRMachineState *spapr)
{
- return DIV_ROUND_UP(max_cpus * spapr->vsmt, smp_threads);
+ return spapr_vcpu_id(spapr, max_cpus)
}
static void xics_system_init(MachineState *machine, int nr_irqs, Error **errp)
