diff options
author | Daniel Henrique Barboza <danielhb413@gmail.com> | 2021-04-08 17:40:48 -0300 |
---|---|---|
committer | David Gibson <david@gibson.dropbear.id.au> | 2021-05-04 11:41:25 +1000 |
commit | 5642e4513e60b46473422902d7cc34e894e8793d (patch) | |
tree | 7e65ad464fa0e32cfab0d0043596adc0530abd89 | |
parent | a7913d5e3fb35572f9ff033e7c1c7f1af6f5f0f7 (diff) |
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>
-rw-r--r-- | hw/ppc/spapr.c | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c index fd53615df0..b37ceb8ee8 100644 --- a/hw/ppc/spapr.c +++ b/hw/ppc/spapr.c @@ -4487,7 +4487,16 @@ 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; |