Re: [PATCH] kvm: x86: Increase MAX_VCPUS to 710

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Igor Mammedov <imammedo@redhat.com> writes:

> On Wed, 01 Sep 2021 10:02:18 +0200
> Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>> 
>> > Support for 710 VCPUs has been tested by Red Hat since RHEL-8.4.
>> > Increase KVM_MAX_VCPUS and KVM_SOFT_MAX_VCPUS to 710.
>> >
>> > For reference, visible effects of changing KVM_MAX_VCPUS are:
>> > - KVM_CAP_MAX_VCPUS and KVM_CAP_NR_VCPUS will now return 710 (of course)
>> > - Default value for CPUID[HYPERV_CPUID_IMPLEMENT_LIMITS (00x40000005)].EAX
>> >   will now be 710
>> > - Bitmap stack variables that will grow:
>> >   - At kvm_hv_flush_tlb()  kvm_hv_send_ipi():
>> >     - Sparse VCPU bitmap (vp_bitmap) will be 96 bytes long
>> >     - vcpu_bitmap will be 92 bytes long
>> >   - vcpu_bitmap at bioapic_write_indirect() will be 92 bytes long
>> >     once patch "KVM: x86: Fix stack-out-of-bounds memory access
>> >     from ioapic_write_indirect()" is applied
>> >
>> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
>> > ---
>> >  arch/x86/include/asm/kvm_host.h | 4 ++--
>> >  1 file changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> > index af6ce8d4c86a..f76fae42bf45 100644
>> > --- a/arch/x86/include/asm/kvm_host.h
>> > +++ b/arch/x86/include/asm/kvm_host.h
>> > @@ -37,8 +37,8 @@
>> >  
>> >  #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
>> >  
>> > -#define KVM_MAX_VCPUS 288
>> > -#define KVM_SOFT_MAX_VCPUS 240
>> > +#define KVM_MAX_VCPUS 710  
>> 
>> Out of pure curiosity, where did 710 came from? Is this some particular
>> hardware which was used for testing (weird number btw). Should we maybe
>> go to e.g. 1024 for the sake of the beauty of powers of two? :-)
>> 
>> > +#define KVM_SOFT_MAX_VCPUS 710  
>> 
>> Do we really need KVM_SOFT_MAX_VCPUS which is equal to KVM_MAX_VCPUS?
>> 
>> Reading 
>> 
>> commit 8c3ba334f8588e1d5099f8602cf01897720e0eca
>> Author: Sasha Levin <levinsasha928@gmail.com>
>> Date:   Mon Jul 18 17:17:15 2011 +0300
>> 
>>     KVM: x86: Raise the hard VCPU count limit
>> 
>> the idea behind KVM_SOFT_MAX_VCPUS was to allow developers to test high
>> vCPU numbers without claiming such configurations as supported.
>> 
>> I have two alternative suggestions:
>> 1) Drop KVM_SOFT_MAX_VCPUS completely.
>> 2) Raise it to a higher number (e.g. 2048)
>> 
>> >  #define KVM_MAX_VCPU_ID 1023  
>> 
>> 1023 may not be enough now. I rememeber there was a suggestion to make
>> max_vcpus configurable via module parameter and this question was
>> raised:
>> 
>> https://lore.kernel.org/lkml/878s292k75.fsf@vitty.brq.redhat.com/
>> 
>> TL;DR: to support EPYC-like topologies we need to keep
>>  KVM_MAX_VCPU_ID = 4 * KVM_MAX_VCPUS
>
> VCPU_ID (sequential 0-n range) is not APIC ID (sparse distribution),
> so topology encoded in the later should be orthogonal to VCPU_ID.

Why do we even have KVM_MAX_VCPU_ID which is != KVM[_SOFT]_MAX_VCPUS
then?

KVM_MAX_VCPU_ID is only checked in kvm_vm_ioctl_create_vcpu() which
passes 'id' down to kvm_vcpu_init() which, in its turn, sets
'vcpu->vcpu_id'. This is, for example, returned by kvm_x2apic_id():

static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
{
        return apic->vcpu->vcpu_id;
}

So I'm pretty certain this is actually APIC id and it has topology in
it.

-- 
Vitaly