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Fri, 3 Dec 2021 09:06:11 +0000 (GMT) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2AEFA4C052; Fri, 3 Dec 2021 09:06:11 +0000 (GMT) Received: from [9.171.47.125] (unknown [9.171.47.125]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 3 Dec 2021 09:06:11 +0000 (GMT) Message-ID: Date: Fri, 3 Dec 2021 10:07:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH 1/4] s390x/pci: use a reserved ID for the default PCI group Content-Language: en-US To: Matthew Rosato , Halil Pasic References: <20211202164110.326947-1-mjrosato@linux.ibm.com> <20211202164110.326947-2-mjrosato@linux.ibm.com> <123ecaef-0f76-614c-2f6d-cdb824a84f56@redhat.com> <20211203000655.0c679c8c.pasic@linux.ibm.com> From: Pierre Morel In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: e3xsYp4yJd-YUvTpLCSO4Pjg0sLNgYvE X-Proofpoint-ORIG-GUID: JoijkeNcBhRXNMs5Co0AbPf5Mk8ok4fH X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.790,Hydra:6.0.425,FMLib:17.11.62.513 definitions=2021-12-03_05,2021-12-02_01,2021-12-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 adultscore=0 impostorscore=0 priorityscore=1501 clxscore=1015 spamscore=0 mlxlogscore=999 malwarescore=0 lowpriorityscore=0 mlxscore=0 suspectscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000 definitions=main-2112030057 Received-SPF: pass client-ip=148.163.156.1; envelope-from=pmorel@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.938, RCVD_IN_MSPIKE_H2=-0.001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: farman@linux.ibm.com, David Hildenbrand , cohuck@redhat.com, richard.henderson@linaro.org, qemu-devel@nongnu.org, qemu-s390x@nongnu.org, thuth@redhat.com, borntraeger@linux.ibm.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/3/21 03:25, Matthew Rosato wrote: > On 12/2/21 6:06 PM, Halil Pasic wrote: >> On Thu, 2 Dec 2021 12:11:38 -0500 >> Matthew Rosato wrote: >> >>>> >>>> What happens if we migrate a VM from old to new QEMU? Won't the >>>> guest be >>>> able to observe the change? >>> >>> Yes, technically --  But # itself is not really all that important, it >>> is provided from CLP Q PCI FN to be subsequently used as input into Q >>> PCI FNGRP -- With the fundamental notion being that all functions that >>> share the same group # share the same group CLP info.  Whether the >>> number is, say, 1 or 5 doesn't matter so much. >>> >>> However..  0xF0 and greater are the only values reserved for hypervisor >>> use.  By using 0x20 we run the risk of accidentally conflating simulated >>> devices and real hardware, hence the desire to change it. >>> >>> Is your concern about a migrated guest with a virtio device trying to do >>> a CLP QUERY PCI FNGRP using 0x20 on a new QEMU?  I suppose we could >>> modify 'clp_service_call, case CLP_QUERY_PCI_FNGRP' to silently catch >>> simulated devices trying to use something other than the default group, >>> e.g.: >>> >>> if ((pbdev->fh & FH_SHM_EMUL) && >>>       (pbdev->zpci_fn.pfgid != ZPCI_DEFAULT_FN_GRP)) { >>>           /* Simulated device MUST have default group */ >>>     pbdev->zpci_fn.pfgid = ZPCI_DEFAULT_FN_GRP; >>>     group = s390_group_find(ZPCI_DEFAULT_FN_GRP); >>> } >>> >>> What do you think? >> >> Another option, and in my opinion the cleaner one would be to tie this >> change to a new machine version. That is if a post-change qemu is used >> in compatibility mode, we would still have the old behavior. >> >> What do you think? >> > > The problem there is that the old behavior goes against the architecture > (group 0x20 could belong to real hardware) and AFAIU assigning this new > behavior only to a new machine version means we can't fix old stable > QEMU versions. > > Also, wait a minute -- migration isn't even an option right now, it's > blocked for zpci devices, both passthrough and simulated (see > aede5d5dfc5f 's390x/pci: mark zpci devices as unmigratable') so I say > let's just move to a proper default group now before we potentially > allow migration later. Looks right to me. -- Pierre Morel IBM Lab Boeblingen