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 10BEFC433FE for ; Thu, 18 Nov 2021 15:15:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E618C615E3 for ; Thu, 18 Nov 2021 15:15:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231636AbhKRPSi (ORCPT ); Thu, 18 Nov 2021 10:18:38 -0500 Received: from smtp-out2.suse.de ([195.135.220.29]:51784 "EHLO smtp-out2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229583AbhKRPSh (ORCPT ); Thu, 18 Nov 2021 10:18:37 -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 CB2AD1FD38; Thu, 18 Nov 2021 15:15:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1637248535; 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=815ygcrkHrZO++5vQDLmQslmOWBdzgBrazSgJaUJgCU=; b=tukfil+0Q7iesX8f0AAZZnVk3VaiW53UnbpPyMZTtuQdcJpFBSgWuLRrYAWdCC3rgufTDs 5kkZVk1SisyJDgV527HDRMCf39gTeb6xaLTaBrjq1bFNLrbSA0q2e/ZTjj7YBP+ZfWyFDG TQ/Xou8WwrPI54yNUtGP7p4VtH1mR/Q= 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 51E9F13D43; Thu, 18 Nov 2021 15:15:35 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id JLgREhdulmEwGgAAMHmgww (envelope-from ); Thu, 18 Nov 2021 15:15:35 +0000 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> <20211116141054.17800-5-jgross@suse.com> From: Juergen Gross Subject: Re: [PATCH v3 4/4] x86/kvm: add boot parameter for setting max number of vcpus per guest Message-ID: Date: Thu, 18 Nov 2021 16:15:34 +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="cy1UV5sB4OCe2ei6mQXIIrO6JHN8LxhIZ" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --cy1UV5sB4OCe2ei6mQXIIrO6JHN8LxhIZ Content-Type: multipart/mixed; boundary="zJjxZtNSeFizLk8L4yXlPqhTIRzHT9Jt6"; 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 4/4] x86/kvm: add boot parameter for setting max number of vcpus per guest References: <20211116141054.17800-1-jgross@suse.com> <20211116141054.17800-5-jgross@suse.com> In-Reply-To: --zJjxZtNSeFizLk8L4yXlPqhTIRzHT9Jt6 Content-Type: multipart/mixed; boundary="------------FF34E3C95AE3FD4A9D2728F5" Content-Language: en-US This is a multi-part message in MIME format. --------------FF34E3C95AE3FD4A9D2728F5 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 18.11.21 16:05, Sean Christopherson wrote: > On Thu, Nov 18, 2021, Juergen Gross wrote: >> On 17.11.21 21:57, Sean Christopherson wrote: >>> Rather than makes this a module param, I would prefer to start with t= he below >>> patch (originally from TDX pre-enabling) and then wire up a way for u= serspace to >>> _lower_ the max on a per-VM basis, e.g. add a capability. >> >> The main reason for this whole series is a request by a partner >> to enable huge VMs on huge machines (huge meaning thousands of >> vcpus on thousands of physical cpus). >> >> Making this large number a compile time setting would hurt all >> the users who have more standard requirements by allocating the >> needed resources even on small systems, so I've switched to a boot >> parameter in order to enable those huge numbers only when required. >> >> With Marc's series to use an xarray for the vcpu pointers only the >> bitmaps for sending IRQs to vcpus are left which need to be sized >> according to the max vcpu limit. Your patch below seems to be fine, bu= t >> doesn't help for that case. >=20 > Ah, you want to let userspace define a MAX_VCPUS that goes well beyond = the current > limit without negatively impacting existing setups. My idea of a per-V= M capability Correct. > still works, it would simply require separating the default max from th= e absolute > max, which this patch mostly does already, it just neglects to set an a= bsolute max. >=20 > Which is a good segue into pointing out that if a module param is added= , it needs > to be sanity checked against a KVM-defined max. The admin may be trust= ed to some > extent, but there is zero reason to let userspace set max_vcspus to 4 b= illion. > At that point, it really is just a param vs. capability question. I agree. Capping it at e.g. 65536 would probably be a good idea. > I like the idea of a capability because there are already two known use= cases, > arm64's GIC and x86's TDX, and it could also be used to reduce the kern= el's footprint > for use cases that run large numbers of smaller VMs. >=20 > The other alternative would be to turn KVM_MAX_VCPUS into a Kconfig kno= b. I assume I like combining the capping and a Kconfig knob. So let the distro (or whoever is building the kernel) decide, which is the max allowed value (e.g. above 65536 per default). > the partner isn't running a vanilla distro build and could set it as th= ey see fit. And here you are wrong. They'd like to use standard SUSE Linux (SLE). 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