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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 01 Sep 2021 12:13:55 +0200
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> 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?
I'd say for compat reasons (8c3ba334f85 KVM: x86: Raise the hard VCPU count limit)

qemu warns users that they are out of recommended (tested) limit when
it sees requested maxcpus over soft limit.
See soft_vcpus_limit in qemu.


> 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.
Yep, I mixed it up with cpu_index on QEMU side,
for x86 it fetches actual apic id and feeds that to kvm when creating vCPU.

It looks like KVM_MAX_VCPU_ID (KVM_SOFT_MAX_VCPUS) is essentially
MAX_[SOFT_]APIC_ID which in some places is treated as max number of vCPUs,
so actual max count of vCPUs could be less than that (in case of sparse APIC
IDs /non power of 2 thread|core|whatever count/).