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[91.12.100.159]) by smtp.gmail.com with ESMTPSA id g11sm10720336eje.7.2021.05.03.01.10.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 03 May 2021 01:10:08 -0700 (PDT) Subject: Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too To: Thomas Huth , Markus Armbruster 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> From: David Hildenbrand Organization: Red Hat Message-ID: <3808e5d4-b755-bce5-a25f-2d46ed6a59e7@redhat.com> Date: Mon, 3 May 2021 10:10:08 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <15271277-98c2-3925-5937-b9f04ad376b3@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=david@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, NICE_REPLY_A=-0.001, 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: Christian Borntraeger , qemu-s390x@nongnu.org, Cornelia Huck , Richard Henderson , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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.