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[79.181.55.232]) by smtp.gmail.com with ESMTPSA id f9sm3771173wre.65.2020.06.09.09.41.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Jun 2020 09:41:19 -0700 (PDT) Date: Tue, 9 Jun 2020 12:41:16 -0400 From: "Michael S. Tsirkin" To: Claudio Imbrenda Subject: Re: [PATCH v2 1/1] virtio-ccw: auto-manage VIRTIO_F_IOMMU_PLATFORM if PV Message-ID: <20200609123953-mutt-send-email-mst@kernel.org> 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> <20200609174747.4e300818@ibm-vm> MIME-Version: 1.0 In-Reply-To: <20200609174747.4e300818@ibm-vm> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=mst@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/09 02:41:53 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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 , David Hildenbrand , Richard Henderson , Cornelia Huck , qemu-devel@nongnu.org, Halil Pasic , 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, Jun 09, 2020 at 05:47:47PM +0200, Claudio Imbrenda wrote: > 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. So my suggestion was basically a flag that sets both the CPU model feature and the virtio feature. > 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? >