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Thu, 25 Jun 2020 12:21:30 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BAC044203F; Thu, 25 Jun 2020 12:21:29 +0000 (GMT) Received: from oc2783563651 (unknown [9.145.188.218]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 25 Jun 2020 12:21:29 +0000 (GMT) Date: Thu, 25 Jun 2020 14:21:02 +0200 From: Halil Pasic To: "Daniel P. =?UTF-8?B?QmVycmFuZ8Op?=" Subject: Re: [RFC 0/4] Enable virtio-fs on s390x Message-ID: <20200625142102.1f82ae4d.pasic@linux.ibm.com> In-Reply-To: <20200625101935.GF1009994@redhat.com> References: <20200625100430.22407-1-mhartmay@linux.ibm.com> <20200625101935.GF1009994@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216, 18.0.687 definitions=2020-06-25_05:2020-06-25, 2020-06-25 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxlogscore=999 malwarescore=0 priorityscore=1501 impostorscore=0 spamscore=0 bulkscore=0 cotscore=-2147483648 mlxscore=0 clxscore=1015 lowpriorityscore=0 suspectscore=0 phishscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006250078 Received-SPF: pass client-ip=148.163.158.5; envelope-from=pasic@linux.ibm.com; helo=mx0b-001b2d01.pphosted.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/25 08:13:37 X-ACL-Warn: Detected OS = Linux 3.1-3.10 X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=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: "Michael S. Tsirkin" , Cornelia Huck , qemu-devel@nongnu.org, Marc Hartmayer , Stefan Hajnoczi , =?UTF-8?B?TWFyYy1BbmRyw6k=?= Lureau Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 25 Jun 2020 11:19:35 +0100 Daniel P. Berrangé wrote: > On Thu, Jun 25, 2020 at 12:04:26PM +0200, Marc Hartmayer wrote: > > This RFC is about enabling virtio-fs on s390x. For that we need > > + some shim code (first patch), and we need > > + libvhost-user to deal with virtio endiannes as mandated by the spec. > > > > The second part is trickier, because unlike QEMU we are not certain > > about the guest's native endianness, which is needed to handle the > > legacy-interface appropriately. In fact, this is the reason why just > > RFC. > > > > One of the open questions is whether to build separate versions, one > > for guest little endian and one for guest big endian, or do we want > > something like a command line option? (Digression on the libvirt > > modeling) > > When you talk about big vs little endian, are you referring to TCG > scenarios with mixed host/guest arch, or arches which can support > either endianess, or both ? i guess it doesn't matter actually, as > I think the latter forces a specific answer. > Our primary concern is big endian cpu and little endian virtio because virto 1.0 or better. But since we talk about a lib here, everything is possible. > Considering that some architectures allow the guest OS to flip between > big & little endian as they boot, libvirt cannot know what endianess > the guest is using when it launches virtiofsd. It thus cannot pick > between two different endianness builds of virtiofsd automatically. > This would force the user to tell libvirt what arch the guest is using > at the time they define the guest. This is an undesirable restriction > for use cases where the admin of the guest OS has no direct control > over the host config. > > IOW, I think the only practical answer is to have a single binary that > automagically does the right thing at runtime according to guest > endianess that currently is in use. > The problem is that AFAIK there is no way to figure out what is the right thing at the moment. I guess QEMU should now, but not virtiofsd or whatever userspace virtio device. But I will double check if there is something in the protocol addressing this. Regards, Halil