From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 51F06C432BE for ; Wed, 1 Sep 2021 15:25:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3490B60FDC for ; Wed, 1 Sep 2021 15:25:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343746AbhIAP02 (ORCPT ); Wed, 1 Sep 2021 11:26:28 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:22006 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1343727AbhIAP0Z (ORCPT ); Wed, 1 Sep 2021 11:26:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1630509928; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=aPK/WRSDF8QrZ8XSCsCWCd7jM/z+l8eT9+5P3Qag9/8=; b=Zu27jAa/t5NuAH6IBiZXlmFjO6ipGoh1tKBWzeDA0uZG3FaeWW1W0xxdKI25iqsxeCn+PZ FZXy2/0W5BiiMdIDY6d9E3cPPdYS3rsVpuoZmFcz0GR5IXkOFdQTfLX+s7fF8SeATmUs1u GK5EH0sNPreRlv1b9855x0ifP0Gpy2I= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-65-VcxYa_cGOyyUtktu9xjr2g-1; Wed, 01 Sep 2021 11:25:26 -0400 X-MC-Unique: VcxYa_cGOyyUtktu9xjr2g-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E303018460E4; Wed, 1 Sep 2021 15:25:25 +0000 (UTC) Received: from localhost (unknown [10.22.8.94]) by smtp.corp.redhat.com (Postfix) with ESMTP id AAAFC19C79; Wed, 1 Sep 2021 15:25:25 +0000 (UTC) Date: Wed, 1 Sep 2021 11:25:25 -0400 From: Eduardo Habkost To: Vitaly Kuznetsov Cc: Igor Mammedov , kvm@vger.kernel.org, Paolo Bonzini , Sean Christopherson Subject: Re: [PATCH] kvm: x86: Increase MAX_VCPUS to 710 Message-ID: <20210901152525.g5fnf5ketta3fjhl@habkost.net> References: <20210831204535.1594297-1-ehabkost@redhat.com> <87sfyooh9x.fsf@vitty.brq.redhat.com> <20210901111326.2efecf6e@redhat.com> <87ilzkob6k.fsf@vitty.brq.redhat.com> <20210901153615.296486b5@redhat.com> <875yvknyrj.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <875yvknyrj.fsf@vitty.brq.redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Sep 01, 2021 at 04:42:08PM +0200, Vitaly Kuznetsov wrote: > Igor Mammedov writes: > > > On Wed, 01 Sep 2021 12:13:55 +0200 > > Vitaly Kuznetsov wrote: > > > >> Igor Mammedov writes: > >> > >> > On Wed, 01 Sep 2021 10:02:18 +0200 > >> > Vitaly Kuznetsov wrote: > >> > > >> >> Eduardo Habkost 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 > >> >> > --- > >> >> > 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 > >> >> 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@vitty.brq.redhat.com/ > >> >> > >> >> 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