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Wed, 14 Aug 2024 07:35:02 -0700 (PDT) Date: Wed, 14 Aug 2024 07:35:01 -0700 In-Reply-To: <20240814123715.GB2032816@nvidia.com> Mime-Version: 1.0 References: <20240809160909.1023470-1-peterx@redhat.com> <20240814123715.GB2032816@nvidia.com> Message-ID: Subject: Re: [PATCH 00/19] mm: Support huge pfnmaps From: Sean Christopherson To: Jason Gunthorpe Cc: Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oscar Salvador , Axel Rasmussen , linux-arm-kernel@lists.infradead.org, x86@kernel.org, Will Deacon , Gavin Shan , Paolo Bonzini , Zi Yan , Andrew Morton , Catalin Marinas , Ingo Molnar , Alistair Popple , Borislav Petkov , David Hildenbrand , Thomas Gleixner , kvm@vger.kernel.org, Dave Hansen , Alex Williamson , Yan Zhao Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240814_073505_226209_A9ECB6D2 X-CRM114-Status: GOOD ( 19.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Aug 14, 2024, Jason Gunthorpe wrote: > On Fri, Aug 09, 2024 at 12:08:50PM -0400, Peter Xu wrote: > > Overview > > ======== > > > > This series is based on mm-unstable, commit 98808d08fc0f of Aug 7th latest, > > plus dax 1g fix [1]. Note that this series should also apply if without > > the dax 1g fix series, but when without it, mprotect() will trigger similar > > errors otherwise on PUD mappings. > > > > This series implements huge pfnmaps support for mm in general. Huge pfnmap > > allows e.g. VM_PFNMAP vmas to map in either PMD or PUD levels, similar to > > what we do with dax / thp / hugetlb so far to benefit from TLB hits. Now > > we extend that idea to PFN mappings, e.g. PCI MMIO bars where it can grow > > as large as 8GB or even bigger. > > FWIW, I've started to hear people talk about needing this in the VFIO > context with VMs. > > vfio/iommufd will reassemble the contiguous range from the 4k PFNs to > setup the IOMMU, but KVM is not able to do it so reliably. Heh, KVM should very reliably do the exact opposite, i.e. KVM should never create a huge page unless the mapping is huge in the primary MMU. And that's very much by design, as KVM has no knowledge of what actually resides at a given PFN, and thus can't determine whether or not its safe to create a huge page if KVM happens to realize the VM has access to a contiguous range of memory.