Up to this patch, 'max_cpus' value is hardcoded to 1024 (commit 6244bb7e5811). In theory this patch would simply bump it to 2048, since it's the default NR_CPUS kernel setting for ppc64 servers nowadays, but the whole mechanic of MachineClass:max_cpus is flawed for the pSeries machine. The two supported accelerators, KVM and TCG, can live without it.
TCG guests don't have a theoretical limit. The user must be free to emulate as many CPUs as the hardware is capable of. And even if there were a limit, max_cpus is not the proper way to report it since it's a common value checked by SMP code in machine_smp_parse() for KVM as well. For KVM guests, the proper way to limit KVM CPUs is by host configuration via NR_CPUS, not a QEMU hardcoded value. There is no technical reason for a pSeries QEMU guest to forcefully stay below NR_CPUS. This hardcoded value also disregard hosts that might have a lower NR_CPUS limit, say 512. In this case, machine.c:machine_smp_parse() will allow a 1024 value to pass, but then kvm_init() will complain about it because it will exceed NR_CPUS: Number of SMP cpus requested (1024) exceeds the maximum cpus supported by KVM (512) A better 'max_cpus' value would consider host settings, but MachineClass::max_cpus is defined well before machine_init() and kvm_init(). We can't check for KVM limits because it's too soon, so we end up making a guess. This patch makes MachineClass:max_cpus settings innocuous by setting it to INT32_MAX. machine.c:machine_smp_parse() will not fail the verification based on max_cpus, letting kvm_init() do the checking with actual host settings. And TCG guests get to do whatever the hardware is capable of emulating. Signed-off-by: Daniel Henrique Barboza <danielhb...@gmail.com> --- hw/ppc/spapr.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c index 73a06df3b1..d6a67da21f 100644 --- a/hw/ppc/spapr.c +++ b/hw/ppc/spapr.c @@ -4482,7 +4482,16 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data) mc->init = spapr_machine_init; mc->reset = spapr_machine_reset; mc->block_default_type = IF_SCSI; - mc->max_cpus = 1024; + + /* + * Setting max_cpus to INT32_MAX. Both KVM and TCG max_cpus values + * should be limited by the host capability instead of hardcoded. + * max_cpus for KVM guests will be checked in kvm_init(), and TCG + * guests are welcome to have as many CPUs as the host are capable + * of emulate. + */ + mc->max_cpus = INT32_MAX; + mc->no_parallel = 1; mc->default_boot_order = ""; mc->default_ram_size = 512 * MiB; -- 2.30.2