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<6c13dd5e0b7650abc008aa3b8981786f094efc73.camel@linux.ibm.com> Subject: Re: [PATCH v3 2/3] vfio/ism: Implement 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: <20260310132015.6e4453be@shazbot.org> References: <20260305-vfio_pci_ism-v3-0-1217076c81d9@linux.ibm.com> <20260305-vfio_pci_ism-v3-2-1217076c81d9@linux.ibm.com> <20260310132015.6e4453be@shazbot.org> Autocrypt: addr=schnelle@linux.ibm.com; prefer-encrypt=mutual; keydata=mQINBGHm3M8BEAC+MIQkfoPIAKdjjk84OSQ8erd2OICj98+GdhMQpIjHXn/RJdCZLa58k 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lowpriorityscore=0 priorityscore=1501 suspectscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2602130000 definitions=main-2603110110 On Tue, 2026-03-10 at 13:20 -0600, Alex Williamson wrote: > On Thu, 05 Mar 2026 13:20:14 +0100 > Julian Ruess wrote: >=20 > > Add a vfio_pci variant driver for the s390-specific Internal Shared > > Memory (ISM) devices used for inter-VM communication. > >=20 > > This enables the development of vfio-pci-based user space drivers for > > ISM devices. > >=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, a= s > > the ISM device relies on the PCI store block (PCISTB) instruction to > > perform write operations. > >=20 > > Stores are not fragmented, therefore one ioctl corresponds to exactly > > one PCISTB instruction. User space must ensure to not write more than > > 4096 bytes at once to an ISM BAR which is the maximum payload of the > > PCISTB instruction. > >=20 > > Reviewed-by: Niklas Schnelle > > Signed-off-by: Julian Ruess > > --- > >=20 --- snip --- > >=20 > > +static ssize_t ism_vfio_pci_do_io_r(struct vfio_pci_core_device *vdev, > > + char __user *buf, loff_t off, size_t count, > > + int bar) > > +{ > > + struct zpci_dev *zdev =3D to_zpci(vdev->pdev); > > + ssize_t ret, done =3D 0; > > + u64 req, length, tmp; > > + > > + while (count) { > > + if (count >=3D 8 && IS_ALIGNED(off, 8)) > > + length =3D 8; > > + else if (count >=3D 4 && IS_ALIGNED(off, 4)) > > + length =3D 4; > > + else if (count >=3D 2 && IS_ALIGNED(off, 2)) > > + length =3D 2; > > + else > > + length =3D 1; > > + req =3D ZPCI_CREATE_REQ(READ_ONCE(zdev->fh), bar, length); > > + /* > > + * Use __zpci_load() to bypass automatic use of PCI MIO instructions > > + * which are not supported on ISM devices > > + */ > > + ret =3D __zpci_load(&tmp, req, off); > > + if (ret) > > + return ret; > > + if (copy_to_user(buf, &tmp, length)) >=20 > Is there an endian issue here for 1/2/4 byte reads? zpci_read_single() > uses a u64 target for zpci_load(), but then casts the result into the > destination buffer to place the low-order bytes. AIUI, copy_to_user() > would start from the high-order bytes of the u64. Great find Alex, thank you! One should think that us s390 people would be the ones extra careful with Endianess. I did a bit of digging how this slipped through our review process and it seems we actually had it right up to our internal v5 using a macro generated ism_read##size() and using a temporary u##size. Then we messed up pulling the copy_to_user() out of the individual ism_read() macros which then got eliminated still without noticing the endianess issue. We actually also don't need the sizes < 8 in the current user-space driver so didn't notice in testing either. And as a side note LLM review also didn't catch it. So unless vfio-pci requires reads < 8 bytes to work we'd opt to just support the 8 byte case. >=20 > > + return -EFAULT; > > + count -=3D length; > > + done +=3D length; > > + off +=3D length; > > + buf +=3D length; > > + } > > + return done; > > +} --- snip --- > > + > > +static int ism_vfio_pci_probe(struct pci_dev *pdev, > > + const struct pci_device_id *id) > > +{ > > + struct ism_vfio_pci_core_device *ivpcd; > > + struct zpci_dev *zdev =3D to_zpci(pdev); > > + int ret; > > + > > + ivpcd =3D vfio_alloc_device(ism_vfio_pci_core_device, core_device.vde= v, > > + &pdev->dev, &ism_pci_ops); > > + if (IS_ERR(ivpcd)) > > + return PTR_ERR(ivpcd); > > + > > + store_block_cache =3D kmem_cache_create("store_block_cache", > > + zdev->maxstbl, 0, 0, NULL); > > + if (!store_block_cache) > > + return -ENOMEM; >=20 > ivpcd is leaked here, we need a vfio_put_device(). Thanks, >=20 > Alex Good find again. For completeness we also noticed another issue here in internal review. The store_block_cache gets replaced and the old one leaked when multiple devices are used with this driver. So we'll likely go to a per-device kvmem_cache in v4. >=20 > > + > > + dev_set_drvdata(&pdev->dev, &ivpcd->core_device); > > + ret =3D vfio_pci_core_register_device(&ivpcd->core_device); > > + if (ret) { > > + kmem_cache_destroy(store_block_cache); > > + vfio_put_device(&ivpcd->core_device.vdev); > > + } > > + > > + return ret; > > +} > >=20