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<467237b118aff01c95486afb733a79220f732388.camel@linux.ibm.com> Subject: Re: [PATCH v4 2/4] s390/pci: store DMA offset in bus_dma_region From: Niklas Schnelle To: Matthew Rosato , joro@8bytes.org, will@kernel.org, robin.murphy@arm.com, gerald.schaefer@linux.ibm.com Cc: hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, svens@linux.ibm.com, borntraeger@linux.ibm.com, farman@linux.ibm.com, clegoate@redhat.com, jgg@nvidia.com, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Halil Pasic Date: Wed, 12 Feb 2025 17:15:47 +0100 In-Reply-To: <52a8f09c-4a59-4439-9738-58f040fed0a5@linux.ibm.com> References: <20250207205335.473946-1-mjrosato@linux.ibm.com> <20250207205335.473946-3-mjrosato@linux.ibm.com> <8380fe8052c9b972a8350394cb6655f6a9683d29.camel@linux.ibm.com> <52a8f09c-4a59-4439-9738-58f040fed0a5@linux.ibm.com> Autocrypt: addr=schnelle@linux.ibm.com; prefer-encrypt=mutual; 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X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 mlxscore=0 clxscore=1015 adultscore=0 mlxlogscore=611 phishscore=0 bulkscore=0 suspectscore=0 spamscore=0 impostorscore=0 malwarescore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2501170000 definitions=main-2502120120 On Wed, 2025-02-12 at 10:23 -0500, Matthew Rosato wrote: > > > +static void pci_dma_range_setup(struct pci_dev *pdev) > > > +{ > > > + struct zpci_dev *zdev =3D to_zpci(pdev); > > > + struct bus_dma_region *map; > > > + > > > + map =3D kzalloc(sizeof(*map), GFP_KERNEL); > > > + if (!map) > > > + return; > > > + > > > + map->cpu_start =3D 0; > > > + map->dma_start =3D PAGE_ALIGN(zdev->start_dma); > > > + map->size =3D max(PAGE_ALIGN_DOWN(zdev->end_dma + 1) - map->dma_sta= rt, 0); > >=20 > > Ugh, this is my fault as I suggested it, but this max() doesn't work > > here. The zdev->end_dma is unsigned and so is map->dma_start so if the > > former is smaller underflow will occur and the max() won't save us. > > It's largely a theoretical issue since zdev->end_dma + 1 should always > > be larger than zdev->start_dma, but now the max() looks like we thought > > of that, but then it doesn't work. > >=20 > > If we handle it maybe just go with: > >=20 > > aligned_end =3D PAGE_ALIGN_DOWN(zdev->end_dma + 1); > > if (aligned_end >=3D map->dma_start) > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0map->size =3D aligned_end - map->dma_star= t; > > else > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0map->size =3D 0; > >=20 >=20 >=20 > Given that it's not really something that's supposed to happen, would it = make sense then to add a=20 >=20 > WARN_ON_ONCE(map->size =3D=3D 0); >=20 > At the end of this? Yes that makes sense to me