On Wed, Feb 14, 2018 at 08:41:03PM +0100, Greg Kurz wrote: > 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.
No, not really. Weird topologies are still allowed on old machine
types for backwards compatibility, and we shouldn't break that. I
like the idea of consolidating this calculation, but we can't do it by
just breaking the older machines (at least not until they're formally
deprecated).
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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