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[159.2.239.150]) by smtp.gmail.com with ESMTPSA id af79cd13be357-945d4844991sm278199585a.28.2026.11.04.11.31.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Nov 2026 11:31:39 -0800 (PST) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1xOghv-0000000HG9b-0zIw; Wed, 04 Nov 2026 15:31:39 -0400 Date: Wed, 4 Nov 2026 15:31:39 -0400 From: Jason Gunthorpe To: Nicolin Chen Cc: "Aneesh Kumar K.V (Arm)" , iommu@lists.linux.dev, Alex Williamson , Alexey Kardashevskiy , Bjorn Helgaas , Catalin Marinas , Jacob Pan , Joerg Roedel , Jonathan Cameron , Jonathan Hunter , Kevin Tian , Krishna Reddy , Lukas Wunner , Robin Murphy , Samuel Ortiz , Shameer Kolothum , Steven Price , Suravee Suthikulpanit , Suzuki K Poulose , Thierry Reding , Vasant Hegde , Will Deacon , Xu Yilun , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, linux-tegra@vger.kernel.org, Jonathan Corbet , Shuah Khan , Randy Dunlap Subject: Re: [PATCH v8 13/15] iommufd/viommu: Allow vIOMMUs without a parent HWPT Message-ID: <20261104193139.GB3889730@ziepe.ca> References: <20261104161632.3547423-1-aneesh.kumar@kernel.org> <20261104161632.3547423-14-aneesh.kumar@kernel.org> X-Mailing-List: linux-kernel@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-Disposition: inline In-Reply-To: On Wed, Nov 04, 2026 at 11:03:41AM -0800, Nicolin Chen wrote: > On Wed, Nov 04, 2026 at 09:46:30PM +0530, Aneesh Kumar K.V (Arm) wrote: > > VIOMMU_ALLOC currently requires a nesting parent HWPT_PAGING even when > > the selected vIOMMU implementation has no use for one. This would force > > implementations such as the following Arm Realm vIOMMU to create an > > unused parent. > > IIRC, MSI on ARM still goes through the NS pathway. So, it wouldn't > be "unused" if the device attaches to it (or its nested hwpts)? It is unused by the RMM VIOMMU. I think that is what is Aneesh is explaining. > By dropping the nesting parent from VIOMMU, it just simplifies the > attachment to an S1 paging hwpt, right? Yes. The VMM must always maintain a S1 paging attachment that is the identity map of the VM on all devices, same action as when there is no viommu. Jason