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<87b6394f8fc587281e310fdc84d7b5fa0135f387.camel@linux.ibm.com> Subject: Re: [PATCH v4 0/3] vfio/pci: Introduce vfio_pci driver for ISM devices From: Niklas Schnelle To: Alex Williamson , Julian Ruess Cc: wintera@linux.ibm.com, ts@linux.ibm.com, oberpar@linux.ibm.com, gbayer@linux.ibm.com, Jason Gunthorpe , Yishai Hadas , Shameer Kolothum , Kevin Tian , mjrosato@linux.ibm.com, alifm@linux.ibm.com, raspl@linux.ibm.com, hca@linux.ibm.com, agordeev@linux.ibm.com, gor@linux.ibm.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-pci@vger.kernel.org In-Reply-To: <20260316130305.21e015ef@shazbot.org> References: <20260313-vfio_pci_ism-v4-0-4765ae056f71@linux.ibm.com> <20260313094127.74aa0767@shazbot.org> <20260316130305.21e015ef@shazbot.org> Autocrypt: addr=schnelle@linux.ibm.com; prefer-encrypt=mutual; keydata=mQINBGHm3M8BEAC+MIQkfoPIAKdjjk84OSQ8erd2OICj98+GdhMQpIjHXn/RJdCZLa58k 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a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=VnNF1IyMAAAA:8 a=MsBGacowydRqnMt2wnQA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMzE3MDA4NSBTYWx0ZWRfXyJ1GKTM45WZx QBdgmdHidQs/tL2UOYOAnE0VC1l986j1NZsAWJgQ7KLY+LUNmOQ+VafM+KZKiLcK4LEV0B3pbGH surt50Oey2cGbiRXFwG62pYRdl/SLiY8s0FBlExcBCBAhrDV0lCSwfL6EmxEjPGXmV3ZKBWFZgs 9366qeJ8w8Pj+pfAvbiRCqM65Cwb/TryLVHKNiOWVa09Gn1eUIOlKL4IkfyHtmsTqdnlS3x8LC+ nf4HShJ9Bp36LcCQ6SCYXlvIJyT2/B570JEJgEIYUEbJW/KRrfZc/U4J2wpcGSdK7lUCP4kJ6uk knq7rVF0DflQFcpXR+OB4FTw7fK176E+s/X75Ex0PeAtU0Ie2srqMnBIXgA2zGB2BOg/EoSCloi 1KRuOU95yc5wijSw7FoEeMOo5t0gr3PepoHVMiiuILxaDnwxHmS7nOiWIblhpadK3CRuzYhq/hF qFUuyvL8S4Ruxx1Sn6w== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-03-17_01,2026-03-16_06,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 lowpriorityscore=0 priorityscore=1501 suspectscore=0 bulkscore=0 spamscore=0 impostorscore=0 malwarescore=0 adultscore=0 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2603050001 definitions=main-2603170085 On Mon, 2026-03-16 at 13:03 -0600, Alex Williamson wrote: > On Mon, 16 Mar 2026 13:33:04 +0100 > "Julian Ruess" wrote: >=20 > > On Fri Mar 13, 2026 at 4:41 PM CET, Alex Williamson wrote: > > > On Fri, 13 Mar 2026 15:40:27 +0100 > > > Julian Ruess wrote: > > > =20 --- snip --- > >=20 > > Hi Alex, > >=20 > > thanks for the quick feedback! > >=20 > > We are currently developing this extension for a non=E2=80=91QEMU vfio = user space > > driver. Reads smaller than 8 bytes are theoretically valid, but they ar= e not > > used by this driver nor the existing in-kernel driver at the moment. We= could > > extend this in the future if needed. > >=20 > > vfio=E2=80=91pci based PCI pass-through of the ISM device is already po= ssible without > > this extension. In that case, the ISM driver in the guest kernel access= es the > > BARs directly through hardware virtualization, without using the new ac= cess > > routines provided by this variant driver. >=20 > Hi Julian, >=20 > The cover letter argues a secondary use case with QEMU, especially in a > nested environment. The ISM range appears to be an interface to a > variety of device types, console and block are noted. It's also noted > in the implementation that the z/Architecture allows sub-8-byte access. >=20 > I think we need to be cautious that the existence of this driver makes > it available for use with QEMU and other VMMs. In the case of QEMU > vfio_region_ops will allow single-byte access by default. >=20 > The restricted access width is positioned as a simplification here, but > it needs to be evaluated against all the use cases. Unless we're 100% > sure none of those use cases rely on sub-8-byte accesses, we might be > setting ourselves up for hacks later to fix or detect partial access > support. >=20 > I'll leave it to IBM folks to determine if this is indeed a > simplification for long term support of all use cases and not a short > term fix for the short term use case. Thanks, >=20 > Alex Hi Alex, Thank you for the insights and expertise it is highly appreciated. Your reasoning makes sense to me and I agree this simplification looks like it may be ok for now but could cause more trouble than it is worth later and there's really no reason not to just support < 8 byte accesses too. One thing I'd like to explain though since you mention ISM potentially being an interface to different device types. This part of the cover letter is easy to misunderstand since we haven't yet send out patches opening up that direction. There is only one ISM device and even future versions would likely just iterate but serve the same purpose. The multiple device types including consoles and block devices will not replace this ISM device but rather sit on top of a virtual bus called the ISM Peer Bus where the ISM device will serve as a communication channel between two Linux instances connected via an ISM device. Specifically in the current design there will be a vfio-pci based user- space daemon/driver on one Linux instance that provides virtual consoles and block devices for other Linux instances on the other side of ISM based communication channels. Those Linux instances will use kernel integrated drivers for the console and block devices. Currently there are no plans for passing these devices through to guests since we can also just pass-through an ISM device and ISM devices being virtual we're not limited in how many we can have. Thanks, Niklas