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Tue, 20 Oct 2026 11:13:28 -0400 (EDT) Date: Tue, 20 Oct 2026 09:13:23 -0600 From: Alex Williamson To: Ryan Roberts Cc: Jason Gunthorpe , Greg Kroah-Hartman , Arnd Bergmann , Catalin Marinas , Will Deacon , Mark Rutland , Marc Zyngier , Oliver Upton , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Jean-Philippe Brucker , Oded Gabbay , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-doc@vger.kernel.org, alex@shazbot.org Subject: Re: [RFC PATCH v2 12/15] vfio: Support VFIO devices that access the CPU MMU Message-ID: <20261020091323.3c51258e@shazbot.org> In-Reply-To: References: <20261015142552.4128028-1-ryan.roberts@arm.com> <20261015142552.4128028-13-ryan.roberts@arm.com> <20261015164421.GD13920@nvidia.com> <95e0f9bd-0203-4c7a-9d09-622593bb3d4c@arm.com> <20261020140944.GA3008132@nvidia.com> X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-pc-linux-gnu) X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 20 Oct 2026 16:06:37 +0100 Ryan Roberts wrote: > On 20/10/2026 15:09, Jason Gunthorpe wrote: > > On Tue, Oct 20, 2026 at 12:51:26PM +0100, Ryan Roberts wrote: > >>> However.. you still have to affirm which single mm is the mm that the > >>> device is going to be doing DMA against. The process the opens the fd > >>> should not be forced to be the mm that it gets linked to. > >> > >> What is the mm representing here? Ultimately we want to install the KVM > >> instance's stage2 pgtables and (pinned) VMID. We currently get that via the > >> bound KVM instance (which looks like it still works using the cdev interface?) > >> > >> The guest will then install the stage1 pgtables and (pinned) ASID associated > >> with the relevant (guest) mm_struct. > >> > >> Do we _need_ an mm association in the host? > > > > Ah.. I see, I didn't quite follow that. OK.. Ah everyone else doing > > this is using iommufd with a viommu to establish their connection > > between device an KVM. eg Jacob Pan's series. That's a probably a > > better idea to stay aligned with the common path. Though your reasons > > are a little different.. > > Just to check, you're referring to [1]? > > I was under the impression that I could open the VFIO device through the cdev > interface, then bind it to the KVM instance via KVM_DEV_VFIO_FILE_ADD... Why do > I need an iommufd at all? The cdev is not unlocked to actually access the device until it's bound to iommufd. Thanks, Alex