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[159.2.73.22]) by smtp.gmail.com with ESMTPSA id af79cd13be357-8d2a864b516sm733755685a.37.2026.04.04.11.53.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 04 Apr 2026 11:53:31 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1w967e-0000000CyRZ-3Zuw; Sat, 04 Apr 2026 15:53:30 -0300 Date: Sat, 4 Apr 2026 15:53:30 -0300 From: Jason Gunthorpe To: Dan Williams Cc: mhonap@nvidia.com, alwilliamson@nvidia.com, jonathan.cameron@huawei.com, dave.jiang@intel.com, alejandro.lucero-palau@amd.com, dave@stgolabs.net, alison.schofield@intel.com, vishal.l.verma@intel.com, ira.weiny@intel.com, dmatlack@google.com, shuah@kernel.org, yishaih@nvidia.com, skolothumtho@nvidia.com, kevin.tian@intel.com, ankita@nvidia.com, vsethi@nvidia.com, cjia@nvidia.com, targupta@nvidia.com, zhiw@nvidia.com, kjaju@nvidia.com, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2 16/20] vfio/cxl: Register regions with VFIO layer Message-ID: <20260404185330.GD2551565@ziepe.ca> References: <20260401143917.108413-1-mhonap@nvidia.com> <20260401143917.108413-17-mhonap@nvidia.com> <69d0169115c06_1b0cc6100a5@dwillia2-mobl4.notmuch> 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-Disposition: inline In-Reply-To: <69d0169115c06_1b0cc6100a5@dwillia2-mobl4.notmuch> On Fri, Apr 03, 2026 at 12:35:45PM -0700, Dan Williams wrote: > > + /* > > + * CXL device memory is RAM, not MMIO. Use memremap() rather than > > Right, CXL.mem is RAM, not MMIO, so I question why is this not being > mapped via a RAM mechanism? Can you explain a bit more about why reusing > vfio MMIO mapping mechanisms is suitable here and what happens when we > get to questions like guest_memfd integration, page conversions, and > large page mapping support? None of that is applicable to VFIO. VFIO owns the entire address space and does not share it with the mm or anything else. > Maybe this is the start of the conversation for something simple, but I > would actually prefer uncached ioremap() if only to avoid all the > coherence management and suitability of MMIO mechanisms questions. The entire thing needs to be mmapable in a cache coherent way since that is what the HW semantic is. If you try to do something else you will break KVM support since it follows the VMA. Jason