Igor Mammedov <imammedo@xxxxxxxxxx> writes: > 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. > That's the reason why we have KVM_SOFT_MAX_VCPUS in addition to KVM_MAX_VCPUS, not why we have KVM_MAX_VCPU_ID :-) > >> 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/). Yes. To avoid the confusion, I'd suggest we re-define KVM_MAX_VCPU_ID as something like: #define KVM_MAX_VCPU_ID_TO_MAX_VCPUS_RATIO 4 #define KVM_MAX_VCPU_ID (KVM_MAX_VCPUS * KVM_MAX_VCPU_ID_TO_MAX_VCPUS_RATIO) and add a comment about sparse APIC IDs/topology. -- Vitaly
