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[2003:d8:2f28:c100:2225:10aa:f247:7b85]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3cf270fd2dfsm685916f8f.3.2025.08.28.14.01.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 28 Aug 2025 14:01:13 -0700 (PDT) Message-ID: Date: Thu, 28 Aug 2025 23:01:12 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: stupid PAT :) To: =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , =?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, 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> <6591105b-969d-44d6-80e1-233c1b84b121@redhat.com> <75aca34d-3557-49e9-a523-bd3244c28190@redhat.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: p1btFDwZMwm12gg3spOoO0uqT7W942t0JIQCrJgQIYw_1756414875 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On 26.08.25 16:27, Thomas Hellström wrote: > Hi, Christian, > > On Tue, 2025-08-26 at 11:56 +0200, Christian König wrote: >> On 26.08.25 11:17, David Hildenbrand wrote: >>> On 26.08.25 11:00, Christian König wrote: >>>> On 26.08.25 10:46, David Hildenbrand wrote: >>>>>>> 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. >>>>>> >>>>>> So what would happen is that we completely clutter the R/B >>>>>> tree used by the PAT with thousands if not millions of >>>>>> entries. >>>>>> >>>>> >>>>> Hm, above you're saying that there is no direct map, but now >>>>> you are saying that the pages were obtained through >>>>> get_free_page()? >>>> >>>> The problem only happens with highmem pages on 32bit kernels. >>>> Those pages are not in the linear mapping. >>> >>> Right, in the common case there is a direct map. >>> >>>> >>>>> I agree that what you describe here sounds suboptimal. But if >>>>> the pages where obtained from the buddy, there surely is a >>>>> direct map -- unless we explicitly remove it :( >>>>> >>>>> If we're talking about individual pages without a directmap, I >>>>> would wonder if they are actually part of a bigger memory >>>>> region that can just be reserved in one go (similar to how >>>>> remap_pfn_range()) would handle it. >>>>> >>>>> Can you briefly describe how your use case obtains these PFNs, >>>>> and how scattered tehy + their caching attributes might be? >>>> >>>> What drivers do is to call get_free_page() or alloc_pages_node() >>>> with the GFP_HIGHUSER flag set. >>>> >>>> For non highmem pages drivers then calls set_pages_wc/uc() which >>>> changes the caching of the linear mapping, but for highmem pages >>>> there is no linear mapping so set_pages_wc() or set_pages_uc() >>>> doesn't work and drivers avoid calling it. >>>> >>>> Those are basically just random system memory pages. So they are >>>> potentially scattered over the whole memory address space. >>> >>> Thanks, that's valuable information. >>> >>> So essentially these drivers maintain their own consistency and PAT >>> is not aware of that. >>> >>> And the real problem is ordinary system RAM. >>> >>> There are various ways forward. >>> >>> 1) We use another interface that consumes pages instead of PFNs, >>> like a >>>    vm_insert_pages_pgprot() we would be adding. >>> >>>    Is there any strong requirement for inserting non-refcounted >>> PFNs? >> >> Yes, there is a strong requirement to insert non-refcounted PFNs. >> >> We had a lot of trouble with KVM people trying to grab a reference to >> those pages even if the VMA had the VM_PFNMAP flag set. >> >>> 2) We add another interface that consumes PFNs, but explicitly >>> states >>>    that it is only for ordinary system RAM, and that the user is >>>    required for updating the direct map. >>> >>>    We could sanity-check the direct map in debug kernels. >> >> I would rather like to see vmf_insert_pfn_prot() fixed instead. >> >> That function was explicitly added to insert the PFN with the given >> attributes and as far as I can see all users of that function expect >> exactly that. >> >>> >>> 3) We teach PAT code in pfnmap_setup_cachemode_pfn() about treating >>> this >>>    system RAM differently. >>> >>> >>> There is also the option for a mixture between 1 and 2, where we >>> get pages, but we map them non-refcounted in a VM_PFNMAP. >>> >>> In general, having pages makes it easier to assert that they are >>> likely ordinary system ram pages, and that the interface is not >>> getting abused for something else. >> >> Well, exactly that's the use case here and that is not abusive at all >> as far as I can see. >> >> What drivers want is to insert a PFN with a certain set of caching >> attributes regardless if it's system memory or iomem. That's why >> vmf_insert_pfn_prot() was created in the first place. >> >> That drivers need to call set_pages_wc/uc() for the linear mapping on >> x86 manually is correct and checking that is clearly a good idea for >> debug kernels. > > So where is this trending? Is the current suggestion to continue > disallowing aliased mappings with conflicting caching modes and enforce > checks in debug kernels? Not sure, it's a mess. The big question is to find out when it is really ok to bypass PAT and when to better let it have a saying. -- Cheers David / dhildenb