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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2A428C433EF for ; Mon, 7 Feb 2022 10:33:20 +0000 (UTC) Received: from localhost ([::1]:36282 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nH1Kh-0006xv-0v for qemu-devel@archiver.kernel.org; Mon, 07 Feb 2022 05:33:19 -0500 Received: from eggs.gnu.org ([209.51.188.92]:52614) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nH0ve-00034d-1A for qemu-devel@nongnu.org; Mon, 07 Feb 2022 05:07:26 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:30377) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nH0va-0004Jm-Gq for qemu-devel@nongnu.org; Mon, 07 Feb 2022 05:07:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644228441; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sJ3QOtukejvI0Tb2Q0CDTGD6ryltBEVDlXxlGUnwZNM=; b=BmGam1voo6sil3fTwTEWI46u2aGJ4lxbIE2yw6ebNCsf0+tbrCFQ6JHGQERYznvTMCCgFp 6TEc8W3eKXUI4TR1OE0x8xwtBhnsxNF7+zSkNU8vjxyESk9Gxy3NDt4b0t4jGA63hCEwkI Vh59ON75FblxSdp7s5z7OND1TexQBl0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-657-i9YLyDzMN8CGe5q-c5PeJg-1; Mon, 07 Feb 2022 05:07:18 -0500 X-MC-Unique: i9YLyDzMN8CGe5q-c5PeJg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A11B9835B4D; Mon, 7 Feb 2022 10:07:16 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.143]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5CCB57B039; Mon, 7 Feb 2022 10:06:51 +0000 (UTC) Date: Mon, 7 Feb 2022 10:06:48 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Krempa Subject: Re: [PATCH v4 2/4] hw/i386: Attach CPUs to machine Message-ID: References: <20220205124526.500158-1-f4bug@amsat.org> <20220205124526.500158-3-f4bug@amsat.org> <20220207091437.52cf36b1@redhat.com> <20220207101843.259f517c@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.1.5 (2021-12-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Yang Zhong , Jean-Philippe Brucker , "Michael S. Tsirkin" , libvir-list@redhat.com, qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Paolo Bonzini , Ani Sinha , Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Feb 07, 2022 at 10:36:42AM +0100, Peter Krempa wrote: > On Mon, Feb 07, 2022 at 10:18:43 +0100, Igor Mammedov wrote: > > On Mon, 7 Feb 2022 09:14:37 +0100 > > Igor Mammedov wrote: > > > > > On Sat, 5 Feb 2022 13:45:24 +0100 > > > Philippe Mathieu-Daudé wrote: > > > > > > > Previously CPUs were exposed in the QOM tree at a path > > > > > > > > /machine/unattached/device[nn] > > > > > > > > where the 'nn' of the first CPU is usually zero, but can > > > > vary depending on what devices were already created. > > > > > > > > With this change the CPUs are now at > > > > > > > > /machine/cpu[nn] > > > > > > > > where the 'nn' of the first CPU is always zero. > > > > > > Could you add to commit message the reason behind the change? > > > > regardless, it looks like unwarranted movement to me > > prompted by livirt accessing/expecting a QOM patch which is > > not stable ABI. I'd rather get it fixed on libvirt side. > > > > If libvirt needs for some reason access a CPU instance, > > it should use @query-hotpluggable-cpus to get a list of CPUs > > (which includes QOM path of already present CPUs) instead of > > hard-codding some 'well-known' path as there is no any guarantee > > that it will stay stable whatsoever. > > I don't disagree with you about the use of hardcoded path, but the way > of using @query-hotpluggable-cpus is not really aligning well for how > it's being used. > > To shed a bit more light, libvirt uses the following hardcoded path > > #define QOM_CPU_PATH "/machine/unattached/device[0]" > > in code which is used to query CPU flags. That code doesn't care at all > which cpus are present but wants to get any of them. So yes, calling > query-hotpluggable-cpus is possible but a bit pointless. > > In general the code probing cpu flags via qom-get is very cumbersome as > it ends up doing ~400 QMP calls at startup of a VM in cases when we deem > it necessary to probe the cpu fully. Yes, that's one QMP call per CPUID feature bit that QEMU knows about. It is a massive performance bottleneck that we need a much better solution for. We really should have raised this with QEMU right away when we found we had this need for 100's of QMP commands. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|