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

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Up to this patch, 'max_cpus' value is hardcoded to 1024 (commit
6244bb7e58). 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>
This commit is contained in:
Daniel Henrique Barboza 2021-04-08 17:40:48 -03:00 committed by David Gibson
parent a7913d5e3f
commit 5642e4513e
1 changed files with 10 additions and 1 deletions
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11
hw/ppc/spapr.c
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@ -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;