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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-61c312c4f88sm6928285a12.24.2025.08.26.05.37.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Aug 2025 05:37:06 -0700 (PDT) Message-ID: Date: Tue, 26 Aug 2025 14:37:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: To: =?UTF-8?Q?Christian_K=C3=B6nig?= , intel-xe@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org, x86@kernel.org Cc: airlied@gmail.com, thomas.hellstrom@linux.intel.com, matthew.brost@intel.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, Lorenzo Stoakes References: <20250820143739.3422-1-christian.koenig@amd.com> <4e5f4ef0-53f1-417e-8f3b-76fd7c64cd23@amd.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: KGgG8VO_BoWP0kOcmYjP-igK16rmujKR3xZfmwL4dBc_1756211828 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On 26.08.25 10:38, Christian König wrote: > On 25.08.25 21:10, David Hildenbrand wrote: >> On 21.08.25 10:10, Christian König wrote: >>> On 20.08.25 17:23, David Hildenbrand wrote: >>>> CCing Lorenzo >>>> >>>> On 20.08.25 16:33, Christian König wrote: >>>>> Hi everyone, >>>>> >>>>> sorry for CCing so many people, but that rabbit hole turned out to be >>>>> deeper than originally thought. >>>>> >>>>> TTM always had problems with UC/WC mappings on 32bit systems and drivers >>>>> often had to revert to hacks like using GFP_DMA32 to get things working >>>>> while having no rational explanation why that helped (see the TTM AGP, >>>>> radeon and nouveau driver code for that). >>>>> >>>>> It turned out that the PAT implementation we use on x86 not only enforces >>>>> the same caching attributes for pages in the linear kernel mapping, but >>>>> also for highmem pages through a separate R/B tree. >>>>> >>>>> That was unexpected and TTM never updated that R/B tree for highmem pages, >>>>> so the function pgprot_set_cachemode() just overwrote the caching >>>>> attributes drivers passed in to vmf_insert_pfn_prot() and that essentially >>>>> caused all kind of random trouble. >>>>> >>>>> An R/B tree is potentially not a good data structure to hold thousands if >>>>> not millions of different attributes for each page, so updating that is >>>>> probably not the way to solve this issue. >>>>> >>>>> Thomas pointed out that the i915 driver is using apply_page_range() >>>>> instead of vmf_insert_pfn_prot() to circumvent the PAT implementation and >>>>> just fill in the page tables with what the driver things is the right >>>>> caching attribute. >>>> >>>> I assume you mean apply_to_page_range() -- same issue in patch subjects. >>> >>> Oh yes, of course. Sorry. >>> >>>> Oh this sounds horrible. Why oh why do we have these hacks in core-mm and have drivers abuse them :( >>> >>> Yeah I was also a bit hesitated to use that, but the performance advantage is so high that we probably can't avoid the general approach. >>> >>>> Honestly, apply_to_pte_range() is just the entry in doing all kinds of weird crap to page tables because "you know better". >>> >>> Exactly that's the problem I'm pointing out, drivers *do* know it better. The core memory management has applied incorrect values which caused all kind of the trouble. >>> >>> The problem is not a bug in PAT nor TTM/drivers but rather how they interact with each other. >>> >>> What I don't understand is why do we have the PAT in the first place? No other architecture does it this way. >> >> Probably because no other architecture has these weird glitches I assume ... skimming over memtype_reserve() and friends there are quite some corner cases the code is handling (BIOS, ACPI, low ISA, system RAM, ...) >> >> >> I did a lot of work on the higher PAT level functions, but I am no expert on the lower level management functions, and in particular all the special cases with different memory types. >> >> IIRC, the goal of the PAT subsystem is to make sure that no two page tables map the same PFN with different caching attributes. > > Yeah, that actually makes sense. Thomas from Intel recently explained the technical background to me: > > Some x86 CPUs write back cache lines even if they aren't dirty and what can happen is that because of the linear mapping the CPU speculatively loads a cache line which is elsewhere mapped uncached. > > So the end result is that the writeback of not dirty cache lines potentially corrupts the data in the otherwise uncached system memory. > > But that a) only applies to memory in the linear mapping and b) only to a handful of x86 CPU types (e.g. recently Intels Luna Lake, AMD Athlons produced before 2004, maybe others). > >> It treats ordinary system RAM (IORESOURCE_SYSTEM_RAM) usually in a special way: no special caching mode. >> >> For everything else, it expects that someone first reserves a memory range for a specific caching mode. >> >> For example, remap_pfn_range()...->pfnmap_track()->memtype_reserve() will make sure that there are no conflicts, to the call memtype_kernel_map_sync() to make sure the identity mapping is updated to the new type. >> >> In case someone ends up calling pfnmap_setup_cachemode(), the expectation is that there was a previous call to memtype_reserve_io() or similar, such that pfnmap_setup_cachemode() will find that caching mode. >> >> >> So my assumption would be that that is missing for the drivers here? > > Well yes and no. > > See the PAT is optimized for applying specific caching attributes to ranges [A..B] (e.g. it uses an R/B tree). But what drivers do here is that they have single pages (usually for get_free_page or similar) and want to apply a certain caching attribute to it. One clarification after staring at PAT code once again: for pages (RAM), the caching attribute is stored in the page flags, not in the R/B tree. If nothing was set, it defaults to _PAGE_CACHE_MODE_WB AFAIKs. -- Cheers David / dhildenb