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[2003:cb:c70d:3a00:d73c:6a8:ca9f:1df7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436e2df9f8bsm25473195e9.26.2025.01.08.08.09.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Jan 2025 08:09:16 -0800 (PST) Message-ID: Date: Wed, 8 Jan 2025 17:09:13 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags To: Jason Gunthorpe Cc: ankita@nvidia.com, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, catalin.marinas@arm.com, will@kernel.org, ryan.roberts@arm.com, shahuang@redhat.com, lpieralisi@kernel.org, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, zhiw@nvidia.com, mochs@nvidia.com, udhoke@nvidia.com, dnigam@nvidia.com, alex.williamson@redhat.com, sebastianene@google.com, coltonlewis@google.com, kevin.tian@intel.com, yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org, gshan@redhat.com, linux-mm@kvack.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20241118131958.4609-1-ankita@nvidia.com> <20241118131958.4609-2-ankita@nvidia.com> <20250106165159.GJ5556@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.01.25 17:51, Jason Gunthorpe wrote: > On Fri, Dec 20, 2024 at 04:42:35PM +0100, David Hildenbrand wrote: >> On 18.11.24 14:19, ankita@nvidia.com wrote: >>> From: Ankit Agrawal >>> >>> Currently KVM determines if a VMA is pointing at IO memory by checking >>> pfn_is_map_memory(). However, the MM already gives us a way to tell what >>> kind of memory it is by inspecting the VMA. >> >> Do you primarily care about VM_PFNMAP/VM_MIXEDMAP VMAs, or also other VMA >> types? > > I think this is exclusively about allowing cachable memory inside a > VM_PFNMAP VMA (created by VFIO) remain cachable inside the guest VM. Thanks! > >>> This patch solves the problems where it is possible for the kernel to >>> have VMAs pointing at cachable memory without causing >>> pfn_is_map_memory() to be true, eg DAX memremap cases and CXL/pre-CXL >>> devices. This memory is now properly marked as cachable in KVM. >> >> Does this only imply in worse performance, or does this also affect >> correctness? I suspect performance is the problem, correct? > > Correctness. Things like atomics don't work on non-cachable mappings. Hah! This needs to be highlighted in the patch description. And maybe this even implies Fixes: etc? > >> Maybe one could just reject such cases (if KVM PFN lookup code not >> already rejects them, which might just be that case IIRC). > > At least VFIO enforces SHARED or it won't create the VMA. > > drivers/vfio/pci/vfio_pci_core.c: if ((vma->vm_flags & VM_SHARED) == 0) That makes a lot of sense for VFIO. > > This is pretty normal/essential for drivers.. > > Are you suggesting the VMA flags should be inspected more? > VM_SHARED/PFNMAP before allowing this? I was wondering if we can safely assume that "device PFNs" can only exist in VM_PFNMAP mappings. Then we can avoid all that pfn_valid() / pfn_is_map_memory() stuff for "obviously not device" memory. I tried to protoype, but have to give up for now; the code is too complicated to make quick progress :) -- Cheers, David / dhildenb