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envelope-from=john.levon@nutanix.com; helo=mx0b-002c1b01.pphosted.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Oct 14, 2025 at 07:49:24AM -0600, Alex Williamson wrote: > > My apologies - we hit the exact same issue internally, but with a much older > > codebase, so I did not realise this could be an upstream problem as well! > > > > We put this down to a bug in the nvidia driver - surely it shouldn't be > > reporting fewer regions than are actually in use. So we applied what we > > thought to be a gross hack of boundary checking, and not using the region > > cache in case it's beyond num_regions. > > > > To put it another way, the header file says: > > > > 217 __u32 num_regions; /* Max region index + 1 */ > > > > If it's not actually the max region index + 1, what are the expected > > semantics of this field, or of region indices more generally? We could not > > find any clear documentation on the topic other than this comment. > > '9' only defines the end of the fixed, pre-defined region indexes for > vfio-pci, ie. VFIO_PCI_NUM_REGIONS. Beyond that, we support device specific > regions. The GFX region is one such device specific region. To be clear, nowhere in the code are we hard-coding 9, we ask the underlying vfio instance for the num_regions value. In this case, it appears that the underlying instance is returning 9, and then trying to use index 9. > There is no fixed limit to the number of regions a device may expose Sure. >, nor is vfio_device_info.num_regions necessarily a static value. Ah - is this documented somewhere? And do you have an example of where this varies over the lifetime of a device currently? IOW, are these devices actually changing the num_regions value they report sometime after initialization (namely, can it change after vfio_device_prepare()) ? Or would querying num_regions still return the original value, and in fact the semantics of the field are "it's not the max region index, but the max region index during initialization" or similar ? > We're currently discussing a uAPI for generating special mappings to a region > that could dynamically increase the reported regions. OK. Probably should rename the field to ->num_init_regions as well as fix up the cache, depending on your replies to the above. thanks john