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Tue, 9 Jun 2020 15:47:50 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C3EF211C04A; Tue, 9 Jun 2020 15:47:49 +0000 (GMT) Received: from ibm-vm (unknown [9.145.5.221]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 9 Jun 2020 15:47:49 +0000 (GMT) Date: Tue, 9 Jun 2020 17:47:47 +0200 From: Claudio Imbrenda To: Halil Pasic Subject: Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV Message-ID: <20200609174747.4e300818@ibm-vm> In-Reply-To: <20200609114130.0ca9190b.pasic@linux.ibm.com> References: <20200514221155.32079-1-pasic@linux.ibm.com> <20200520121507-mutt-send-email-mst@kernel.org> <20200606013217.2cffa3ed.pasic@linux.ibm.com> <20200608181428.3c6f127c.cohuck@redhat.com> <20200608190045.319dd68b.pasic@linux.ibm.com> <20200609084402.35d317ec.cohuck@redhat.com> <20200609114130.0ca9190b.pasic@linux.ibm.com> Organization: IBM X-Mailer: Claws Mail 3.17.5 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216, 18.0.687 definitions=2020-06-09_09:2020-06-09, 2020-06-09 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 mlxscore=0 phishscore=0 clxscore=1011 adultscore=0 mlxlogscore=835 cotscore=-2147483648 suspectscore=0 spamscore=0 priorityscore=1501 malwarescore=0 lowpriorityscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006090115 Received-SPF: pass client-ip=148.163.158.5; envelope-from=imbrenda@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/09 11:42:10 X-ACL-Warn: Detected OS = Linux 3.x [generic] [fuzzy] X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, KHOP_DYNAMIC=0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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 , Boris Fiuczynski , Janosch Frank , Pierre Morel , "Michael S. Tsirkin" , Richard Henderson , Cornelia Huck , David Hildenbrand , qemu-devel@nongnu.org, Christian Borntraeger , qemu-s390x@nongnu.org, Viktor Mihajlovski , David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 9 Jun 2020 11:41:30 +0200 Halil Pasic wrote: [...] > I don't know. Janosch could answer that, but he is on vacation. Adding > Claudio maybe he can answer. My understanding is, that while it might > be possible, it is ugly at best. The ability to do a transition is > indicated by a CPU model feature. Indicating the feature to the guest > and then failing the transition sounds wrong to me. I agree. If the feature is advertised, then it has to work. I don't think we even have an architected way to fail the transition for that reason. What __could__ be done is to prevent qemu from even starting if an incompatible device is specified together with PV. Another option is to disable PV at the qemu level if an incompatible device is present. This will have the effect that trying to boot a secure guest will fail mysteriously, which is IMHO also not too great. do we really have that many incompatible devices?