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[2003:d8:2f38:1d00:657c:2aac:ecf5:5df8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-454a99693f2sm134974555e9.5.2025.07.07.00.32.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Jul 2025 00:32:11 -0700 (PDT) Message-ID: <19e3e53c-71e7-456e-a684-916c994872f7@redhat.com> Date: Mon, 7 Jul 2025 09:32:09 +0200 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 v10 5/6] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags To: ankita@nvidia.com, jgg@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, ddutile@redhat.com, seanjc@google.com Cc: aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, kjaju@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, tabba@google.com, qperret@google.com, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, maobibo@loongson.cn References: <20250705071717.5062-1-ankita@nvidia.com> <20250705071717.5062-6-ankita@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 05.07.25 09:17, ankita@nvidia.com wrote: > From: Ankit Agrawal > > Today KVM forces the memory to either NORMAL or DEVICE_nGnRE > based on pfn_is_map_memory (which tracks whether the device memory > is in the kernel map) and ignores the per-VMA flags that indicates the > memory attributes. The KVM code is thus restrictive and allows only for > the memory that is added to the kernel to be marked as cacheable. > > The device memory such as on the Grace Hopper/Blackwell systems > is interchangeable with DDR memory and retains properties such as > cacheability, unaligned accesses, atomics and handling of executable > faults. This requires the device memory to be mapped as NORMAL in > stage-2. > > Given that the GPU device memory is not added to the kernel (but is rather > VMA mapped through remap_pfn_range() in nvgrace-gpu module which sets > VM_PFNMAP), pfn_is_map_memory() is false and thus KVM prevents such memory > to be mapped Normal cacheable. The patch aims to solve this use case. > > Note when FWB is not enabled, the kernel expects to trivially do > cache management by flushing the memory by linearly converting a > kvm_pte to phys_addr to a KVA, see kvm_flush_dcache_to_poc(). The > cache management thus relies on memory being mapped. Moreover > ARM64_HAS_CACHE_DIC CPU cap allows KVM to avoid flushing the icache > and turns icache_inval_pou() into a NOP. These two capabilities > are thus a requirement of the cacheable PFNMAP feature. Make use of > kvm_arch_supports_cacheable_pfnmap() to check them. > > A cachebility check is made by consulting the VMA pgprot value. > If the pgprot mapping type is cacheable, it is safe to be mapped S2 > cacheable as the KVM S2 will have the same Normal memory type as the > VMA has in the S1 and KVM has no additional responsibility for safety. > Checking pgprot as NORMAL is thus a KVM sanity check. > > No additional checks for MTE are needed as kvm_arch_prepare_memory_region() > already tests it at an early stage during memslot creation. There would > not even be a fault if the memslot is not created. > > CC: Oliver Upton > CC: Sean Christopherson > Suggested-by: Jason Gunthorpe > Suggested-by: Catalin Marinas > Suggested-by: David Hildenbrand > Tested-by: Donald Dutile > Signed-off-by: Ankit Agrawal > --- Reviewed-by: David Hildenbrand -- Cheers, David / dhildenb