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Mon, 3 May 2021 10:23:20 +0200 (CEST) From: Markus Armbruster To: David Hildenbrand Subject: Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too 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> Date: Mon, 03 May 2021 10:23:20 +0200 In-Reply-To: <3808e5d4-b755-bce5-a25f-2d46ed6a59e7@redhat.com> (David Hildenbrand's message of "Mon, 3 May 2021 10:10:08 +0200") Message-ID: <87mttcdy4n.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@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 , Cornelia Huck , 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" 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?