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bh=thxwbgD3pAolcvzTOpYFCeadrmnoPrwIYtMD2EnveOM=; b=gXgeAUCEfQfU/ExgSQtRhm1HXoYoWdxet+fEgo5+WIS4z25QpcGefBNbKMhEzE8jrj9b1G eilaFgpSPUxFDluDsM8YaUWYJv3t8wmMN5J1nlARjo6ctNU3fCb1EsDo4wTU/lbwyi2rFH bb3BzEbF6Z2t9fsd2wX+p6t3vNjXQnY= 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-496-qQ9-DOcUNnGysYc20KU7qQ-1; Mon, 03 May 2021 05:23:33 -0400 X-MC-Unique: qQ9-DOcUNnGysYc20KU7qQ-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 855D2800D62; Mon, 3 May 2021 09:23:32 +0000 (UTC) Received: from gondolin.fritz.box (ovpn-113-109.ams2.redhat.com [10.36.113.109]) by smtp.corp.redhat.com (Postfix) with ESMTP id 83F0160C0F; Mon, 3 May 2021 09:23:27 +0000 (UTC) Date: Mon, 3 May 2021 11:23:25 +0200 From: Cornelia Huck To: David Hildenbrand Subject: Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too Message-ID: <20210503112325.23c0f139.cohuck@redhat.com> In-Reply-To: <18b0bccc-145b-c324-9054-3d64af330a99@redhat.com> References: <20210502174836.838816-1-thuth@redhat.com> <20210502174836.838816-5-thuth@redhat.com> <87r1iofm61.fsf@dusky.pond.sub.org> <15271277-98c2-3925-5937-b9f04ad376b3@redhat.com> <3808e5d4-b755-bce5-a25f-2d46ed6a59e7@redhat.com> <87mttcdy4n.fsf@dusky.pond.sub.org> <20210503111457.6f8e3597.cohuck@redhat.com> <18b0bccc-145b-c324-9054-3d64af330a99@redhat.com> Organization: Red Hat GmbH MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=cohuck@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.697, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Thomas Huth , Richard Henderson , Markus Armbruster , qemu-devel@nongnu.org, Christian Borntraeger , qemu-s390x@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, 3 May 2021 11:17:54 +0200 David Hildenbrand wrote: > On 03.05.21 11:14, Cornelia Huck wrote: > > On Mon, 03 May 2021 10:23:20 +0200 > > Markus Armbruster wrote: > > > >> David Hildenbrand writes: > >> > >>> On 03.05.21 07:17, Thomas Huth wrote: > >>>> On 03/05/2021 06.58, Markus Armbruster wrote: > >>>>> Thomas Huth writes: > >>>>> > >>>>>> Clang unfortunately does not support generating code for the z900 > >>>>>> architecture level and starts with the z10 instead. Thus to be able > >>>>>> to support compiling with Clang, we have to check for the supported > >>>>>> compiler flags. The disadvantage is of course that the bios image > >>>>>> will only run with z10 guest CPUs upwards (which is what most people > >>>>>> use anyway), so just in case let's also emit a warning in that case. > >>>>> > >>>>> What happens when you try to use this bios with an old CPU anyway? > >>>> > >>>> Interesting question. I was expecting the guest to crash since it would be > >>>> using a CPU instruction that is not supported on the old CPU model. But I > >>>> just gave it a try, and there was no crash. The guest booted just fine. > >>>> Either Clang only emits instructions that work with the old z900 anyway, or > >>>> our emulation in QEMU is imprecise and we allow newer instructions to be > >>>> executed on old models, too. > >>> > >>> Yes, that's currently still done. We once thought about disabling that > >>> (there was a patch from Richard), but decided against it because -- back > >>> then -- the default QEMU model was still very basic and would have > >>> essentially disabled all more recent instructions as default. > >>> > >>> We can most probably do that change soon as we have a "fairly new" > >>> default QEMU CPU model. I can glue it to my z14 change. > >> > >> In case this makes the BIOS crash with old CPUs: when a guest refuses to > >> start because the BIOS was compiled the wrong way for it, configure > >> having told you so back then is not a nice user experience. Can we do > >> better, with reasonable effort? > > > > I fear the experience will be as bad as for any guest that is using > > features from a newer cpu level (i.e. random crashes when the guest > > actually tries to use that newer instruction.) > > > > I see two options: > > - Just try to start and hope that it works. > > - Deprecate any cpu model older than z10. > > > > Anyone have a better idea? I don't particularly like any of the two. > > As the default CPU model with new compat machines is >= z13, I wouldn't > lose sleep about this. Even with a broken bios one can still boot an > external kernel+initrd for testing purposes. Yes, I do not see many people running into this problem. Still, I fear it will be hard to figure out what exactly the problem is, when it arises...