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