On Wed, 01 Sep 2021 12:13:55 +0200 Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > Igor Mammedov <imammedo@xxxxxxxxxx> writes: > > > On Wed, 01 Sep 2021 10:02:18 +0200 > > Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > > > >> Eduardo Habkost <ehabkost@xxxxxxxxxx> 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@xxxxxxxxxx> > >> > --- > >> > 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@xxxxxxxxx> > >> 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@xxxxxxxxxxxxxxxxxxxx/ > >> > >> 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/).
