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Mon, 30 Mar 2026 14:09:47 -0400 (EDT) Date: Mon, 30 Mar 2026 12:09:45 -0600 From: Alex Williamson To: Jason Gunthorpe Cc: Niklas Schnelle , Farhan Ali , Julian Ruess , wintera@linux.ibm.com, ts@linux.ibm.com, oberpar@linux.ibm.com, gbayer@linux.ibm.com, Yishai Hadas , Shameer Kolothum , Kevin Tian , mjrosato@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 v8 2/3] vfio/ism: Implement vfio_pci driver for ISM devices Message-ID: <20260330120945.023b2295@shazbot.org> In-Reply-To: <20260330155651.GD246076@ziepe.ca> References: <20260325-vfio_pci_ism-v8-0-ddc504cde914@linux.ibm.com> <20260325-vfio_pci_ism-v8-2-ddc504cde914@linux.ibm.com> <4096cb9d402b1aabc9667d450221cb0922e78d90.camel@linux.ibm.com> <3d6457b9-715a-4cbb-9c69-d8a7e7473c89@linux.ibm.com> <64e3158a441c79c55febead9aac956c31f034fb9.camel@linux.ibm.com> <20260330093646.03b0455f@shazbot.org> <20260330155651.GD246076@ziepe.ca> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.51; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 30 Mar 2026 12:56:51 -0300 Jason Gunthorpe wrote: > On Mon, Mar 30, 2026 at 09:36:46AM -0600, Alex Williamson wrote: > > There's risk involved with changing the default shift. The fear is > > there's userspace drivers that hard code the shift. DPDK was even such > > a user at one point, iirc. Maybe it's ok to break such users, maybe > > there are actually no such users left and it's all FUD at this point. > > Either way, I have a hard time justifying that risk for a single, > > obscure S390 device. > > If we go ahead with that DMABUF series could obscure cases like this > be told to just get a DMABUF FD and then mmap it? Avoid this whole > issue? The ISM device here doesn't support mmap, so that much is still a problem. However, if we imagine a future where we've fully converted to dma-buf for both DMA and CPU access to BARs, then maybe... I don't know how we'd describe a region overflow in the "legacy" ioctls to maintain some consistency between what's exposed through the region versus the dma-buf. Thanks, Alex