On Wed, Sep 01, 2021 at 04:42:08PM +0200, Vitaly Kuznetsov wrote: > 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? :-) 710 wasn't tested with real VMs yet due to userspace limitations that still need to be addressed (specifically, due to SMBIOS 2.1 table size limits). I would be more than happy to set it to 1024 or 2048 if the KVM maintainers agree. :) For reference, RHEL-8.4 is compiled with KVM_MAX_VCPUS=2048, but userspace enforces a 710 VCPU limit. > >> >> > >> >> > +#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) I will send a RFC later proposing we make KVM_MAX_VCPUS configurable by Kconfig, and dropping KVM_SOFT_MAX_VCPUS. > >> >> > >> >> > #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 1024 seems to be enough on all the CPU topologies I have seen, but I can happily implement the 4x rule below, just to be sure. > >> > > >> > 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. I will submit a new version of this patch with a rule like the above. A 4x ratio is very generous, but the only impact of a large KVM_MAX_VCPU_ID is a larger struct kvm_ioapic. Changing KVM_MAX_VCPU_ID from 1024 to 4096 will make struct kvm_ioapic grow from 1628 bytes to 5084 bytes, which I assume is OK. -- Eduardo
