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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE071C433EF for ; Thu, 18 Nov 2021 15:19:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BE1BA61B3A for ; Thu, 18 Nov 2021 15:19:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231779AbhKRPWT (ORCPT ); Thu, 18 Nov 2021 10:22:19 -0500 Received: from smtp-out2.suse.de ([195.135.220.29]:51934 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231443AbhKRPWS (ORCPT ); Thu, 18 Nov 2021 10:22:18 -0500 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 8187E1FD37; Thu, 18 Nov 2021 15:19:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1637248756; h=from:from:reply-to: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=Xf7DkdQhGiWRoeZ1EV9Ru0DqOCR9S/9CCgSkKFVsu1E=; b=uRSezixehLFQs6Hbu0zTDOBK+ONW8XmhFRjKXEHDiu7SWy1AjXmdh+wyGGM5hNLrbVRx6k 12yBvd3AXM+UqpRLDlFCuE+T8yqJs2I4rP9gQGk5srNHHIsGhMNC6tkhk8O0kpXbRqX58W Z27FDhfLjw6EyRyvexel9TTKnNNt/JY= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 0A3E913D43; Thu, 18 Nov 2021 15:19:16 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id /blAAfRulmG/GwAAMHmgww (envelope-from ); Thu, 18 Nov 2021 15:19:16 +0000 Subject: Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits To: Sean Christopherson Cc: kvm@vger.kernel.org, x86@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Jonathan Corbet , Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" References: <20211116141054.17800-1-jgross@suse.com> <20211116141054.17800-2-jgross@suse.com> <7f10b8b4-e753-c977-f201-5ef17a6e81c8@suse.com> <731540b4-e8fc-0322-5aa0-e134bc55a397@suse.com> From: Juergen Gross Message-ID: Date: Thu, 18 Nov 2021 16:19:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="P2IkS8001Jv52jNnxYTynxvUQ3SHNB1Wg" Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --P2IkS8001Jv52jNnxYTynxvUQ3SHNB1Wg Content-Type: multipart/mixed; boundary="0MXfMVWoRZ298st46rlc3jOkPZu6xmN8c"; protected-headers="v1" From: Juergen Gross To: Sean Christopherson Cc: kvm@vger.kernel.org, x86@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Jonathan Corbet , Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" Message-ID: Subject: Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits References: <20211116141054.17800-1-jgross@suse.com> <20211116141054.17800-2-jgross@suse.com> <7f10b8b4-e753-c977-f201-5ef17a6e81c8@suse.com> <731540b4-e8fc-0322-5aa0-e134bc55a397@suse.com> In-Reply-To: --0MXfMVWoRZ298st46rlc3jOkPZu6xmN8c Content-Type: multipart/mixed; boundary="------------321CAFC05EC7AF4C22ED8862" Content-Language: en-US This is a multi-part message in MIME format. --------------321CAFC05EC7AF4C22ED8862 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 18.11.21 16:09, Sean Christopherson wrote: > On Thu, Nov 18, 2021, Juergen Gross wrote: >> On 18.11.21 00:46, Sean Christopherson wrote: >>> On Wed, Nov 17, 2021, Juergen Gross wrote: >>>> On 16.11.21 15:10, Juergen Gross wrote: >>>>> Today the maximum vcpu-id of a kvm guest's vcpu on x86 systems is s= et >>>>> via a #define in a header file. >>>>> >>>>> In order to support higher vcpu-ids without generally increasing th= e >>>>> memory consumption of guests on the host (some guest structures con= tain >>>>> arrays sized by KVM_MAX_VCPU_IDS) add a boot parameter for adding s= ome >>>>> bits to the vcpu-id. Additional bits are needed as the vcpu-id is >>>>> constructed via bit-wise concatenation of socket-id, core-id, etc. >>>>> As those ids maximum values are not always a power of 2, the vcpu-i= ds >>>>> are sparse. >>>>> >>>>> The additional number of bits needed is basically the number of >>>>> topology levels with a non-power-of-2 maximum value, excluding the = top >>>>> most level. >>>>> >>>>> The default value of the new parameter will be 2 in order to suppor= t >>>>> today's possible topologies. The special value of -1 will use the >>>>> number of bits needed for a guest with the current host's topology.= >>>>> >>>>> Calculating the maximum vcpu-id dynamically requires to allocate th= e >>>>> arrays using KVM_MAX_VCPU_IDS as the size dynamically. >>>>> >>>>> Signed-of-by: Juergen Gross >>>> >>>> Just thought about vcpu-ids a little bit more. >>>> >>>> It would be possible to replace the topology games completely by an >>>> arbitrary rather high vcpu-id limit (65536?) and to allocate the mem= ory >>>> depending on the max vcpu-id just as needed. >>>> >>>> Right now the only vcpu-id dependent memory is for the ioapic consis= ting >>>> of a vcpu-id indexed bitmap and a vcpu-id indexed byte array (vector= s). >>>> >>>> We could start with a minimal size when setting up an ioapic and ext= end >>>> the areas in case a new vcpu created would introduce a vcpu-id outsi= de >>>> the currently allocated memory. Both arrays are protected by the ioa= pic >>>> specific lock (at least I couldn't spot any unprotected usage when >>>> looking briefly into the code), so reallocating those arrays shouldn= 't >>>> be hard. In case of ENOMEM the related vcpu creation would just fail= =2E >>>> >>>> Thoughts? >>> >>> Why not have userspace state the max vcpu_id it intends to creates on= a per-VM >>> basis? Same end result, but doesn't require the complexity of reallo= cating the >>> I/O APIC stuff. >>> >> >> And if the userspace doesn't do it (like today)? >=20 > Similar to my comments in patch 4, KVM's current limits could be used a= s the > defaults, and any use case wanting to go beyond that would need an upda= ted > userspace. Exceeding those limits today doesn't work, so there's no AB= I breakage > by requiring a userspace change. Hmm, nice idea. Will look into it. > Or again, this could be a Kconfig knob, though that feels a bit weird i= n this case. > But it might make sense if it can be tied to something in the kernel's = config? Having a Kconfig knob for an absolute upper bound of vcpus should be fine. If someone doesn't like the capability to explicitly let qemu create very large VMs, he/she can still set that upper bound to the normal KVM_MAX_VCPUS value. 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