Re: [PATCH RFC] KVM: x86: Drop arbitraty KVM_SOFT_MAX_VCPUS

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> On 11/11/21 14:47, Vitaly Kuznetsov wrote:
>> KVM_CAP_NR_VCPUS is used to get the "recommended" maximum number of
>> VCPUs and arm64/mips/riscv report num_online_cpus(). Powerpc reports
>> either num_online_cpus() or num_present_cpus(), s390 has multiple
>> constants depending on hardware features. On x86, KVM reports an
>> arbitrary value of '710' which is supposed to be the maximum tested
>> value but it's possible to test all KVM_MAX_VCPUS even when there are
>> less physical CPUs available.
>> 
>> Drop the arbitrary '710' value and return num_online_cpus() on x86 as
>> well. The recommendation will match other architectures and will mean
>> 'no CPU overcommit'.
>> 
>> For reference, QEMU only queries KVM_CAP_NR_VCPUS to print a warning
>> when the requested vCPU number exceeds it. The static limit of '710'
>> is quite weird as smaller systems with just a few physical CPUs should
>> certainly "recommend" less.
>> 
>> Suggested-by: Eduardo Habkost <ehabkost@xxxxxxxxxx>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> Yes, this is a good idea.  We cannot move it entirely to common code due 
> to POWER's handling of secondary threads in hypervisors; still, this is 
> as close as we can get to a common idea of what KVM_CAP_NR_VCPUS means.
>

S390's idea is also different and while I don't understand at all
all these hardware features, KVM_CAP_NR_VCPUS == KVM_CAP_MAX_VCPUS
(afaict). This was the first reason to keep KVM_CAP_NR_VCPUS handling in
arch specific code.

-- 
Vitaly