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

[Igor Mammedov <imammedo@redhat.com>]

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.

> >  /* memory slots that are not exposed to userspace */
> >  #define KVM_PRIVATE_MEM_SLOTS 3  
>