spapr.c: do not use MachineClass::max_cpus to limit CPUs

spapr.c: do not use MachineClass::max_cpus to limit CPUs

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 <danielhb413@gmail.com>
Message-Id: <20210408204049.221802-2-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>