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spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:48330 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldUhw-0000bN-JF for qemu-devel@archiver.kernel.org; Mon, 03 May 2021 05:17:40 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60736) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldUfW-0007Bg-9R for qemu-devel@nongnu.org; Mon, 03 May 2021 05:15:10 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:31929) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldUfU-0005yg-Es for qemu-devel@nongnu.org; Mon, 03 May 2021 05:15:09 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620033307; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ivhBzJxCglYk/WN8qV1DNyck8uFjfmDAqe0NmFtmXrc=; b=N9M3a9jK6UK1bVACGawXzz3ry285W0R3b5ZS9de4TgL6yZkbShgqccwQzxuoKv/H8twNTi PwOLpAhDEjtpHb7BYMtTKLg7Ia2DB1wRKSzqaReIzh17GBHhzK6jRBfo9m1PRDFkeLvX2F ht9xPwBhIxBFyfnbrZR76rVIoPDLAlU= 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-347-kV73bjqwPK6Zo4XS-eMaxA-1; Mon, 03 May 2021 05:15:06 -0400 X-MC-Unique: kV73bjqwPK6Zo4XS-eMaxA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2315D6D249; Mon, 3 May 2021 09:15:05 +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 28C2F5D736; Mon, 3 May 2021 09:14:59 +0000 (UTC) Date: Mon, 3 May 2021 11:14:57 +0200 From: Cornelia Huck To: Markus Armbruster Subject: Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too Message-ID: <20210503111457.6f8e3597.cohuck@redhat.com> In-Reply-To: <87mttcdy4n.fsf@dusky.pond.sub.org> 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> Organization: Red Hat GmbH MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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=216.205.24.124; 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 , David Hildenbrand , Richard Henderson , 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, 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.