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Fri, 13 Mar 2026 11:41:29 -0400 (EDT) Date: Fri, 13 Mar 2026 09:41:27 -0600 From: Alex Williamson To: Julian Ruess Cc: schnelle@linux.ibm.com, 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, alex@shazbot.org Subject: Re: [PATCH v4 0/3] vfio/pci: Introduce vfio_pci driver for ISM devices Message-ID: <20260313094127.74aa0767@shazbot.org> In-Reply-To: <20260313-vfio_pci_ism-v4-0-4765ae056f71@linux.ibm.com> References: <20260313-vfio_pci_ism-v4-0-4765ae056f71@linux.ibm.com> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Fri, 13 Mar 2026 15:40:27 +0100 Julian Ruess wrote: > Hi all, >=20 > This series adds a vfio_pci variant driver for the s390-specific > Internal Shared Memory (ISM) devices used for inter-VM communication > including SMC-D. >=20 > This is a prerequisite for an in-development open-source user space > driver stack that will allow to use ISM devices to provide remote > console and block device functionality. This stack will be part of > s390-tools. >=20 > This driver would also allow QEMU to mediate access to an ISM device, > enabling a form of PCI pass-through even for guests whose hardware > cannot directly execute PCI accesses, such as nested guests. >=20 > On s390, kernel primitives such as ioread() and iowrite() are switched > over from function handle based PCI load/stores instructions to PCI > memory-I/O (MIO) loads/stores when these are available and not > explicitly disabled. Since these instructions cannot be used with ISM > devices, ensure that classic function handle-based PCI instructions are > used instead. >=20 > The driver is still required even when MIO instructions are disabled, as > the ISM device relies on the PCI store=E2=80=91block (PCISTB) instruction= to > perform write operations. >=20 > Thank you, > Julian >=20 > Signed-off-by: Julian Ruess > --- > Changes in v4: > - Fix bug with < 8 byte reads. For code simplicity, only support 8 byte r= eads. Does the ISM device define sub-8-byte accesses as valid? It looks like if pread() doesn't return the desired size QEMU will fill the return with -1. Unless such accesses are classified as undefined by ISM, doesn't that suggest a potential data corruption issue to the guest driver? Thanks, Alex