[PULL,34/46] spapr.c: do not use MachineClass::max_cpus to limit CPUs

Commit Message

David Gibson May 4, 2021, 5:53 a.m. UTC

From: Daniel Henrique Barboza <[e-mail]>

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 <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: David Gibson <[e-mail]>
---
 hw/ppc/spapr.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

Patch

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;