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b=lNXQ710yoQFsoMZV3Pe0hhxBarKC6nobCczUy3MYAVOD7mA1WmuZj6PtiOd6CspZt0Jg+Fe9ZLxYLFuSDh33ZNxhd47i6u57ochBzulX5OXJAK09mCaAZ5bs3NId47i+0i6jJlgiJpfji+N6Gl2Il0ASam+erKnPpcxqo+XoBe8= Authentication-Results: mx.microsoft.com 1; dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL4PR12MB9505.namprd12.prod.outlook.com (2603:10b6:208:591::16) by MN2PR12MB4224.namprd12.prod.outlook.com (2603:10b6:208:1dd::8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.522.12; Wed, 14 Oct 2026 04:53:45 +0000 Received: from BL4PR12MB9505.namprd12.prod.outlook.com ([fe80::73aa:eb8c:a86b:5a83]) by BL4PR12MB9505.namprd12.prod.outlook.com ([fe80::73aa:eb8c:a86b:5a83%5]) with mapi id 15.21.0522.011; Wed, 14 Oct 2026 04:53:45 +0000 Message-ID: <84a84584-404d-4e76-a2fd-65bb4aafab47@amd.com> Date: Wed, 14 Oct 2026 10:23:30 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] iommu/amd: Don't allocate kdump device table from DMA32 To: Matt Fleming , Joerg Roedel Cc: Suravee Suthikulpanit , Ashish Kalra , Sairaj Kodilkar , Ankit Soni , Baoquan He , iommu@lists.linux.dev, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@cloudflare.com, Matt Fleming References: <20261013121944.4060097-1-matt@readmodwrite.com> <20261013121944.4060097-3-matt@readmodwrite.com> Content-Language: en-US From: Vasant Hegde In-Reply-To: <20261013121944.4060097-3-matt@readmodwrite.com> Content-Type: text/plain; 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At the > time, the kdump kernel programmed a copy of the old table into the base > register with the IOMMU still enabled. The 64-bit register is updated > as two 32-bit halves, so the running IOMMU could see a half-written, > invalid table pointer. Keeping both tables below 4G leaves the upper > half zero throughout the update, so the pointer stays valid. > > Since commit 38e5f33ee359 ("iommu/amd: Reuse device table for kdump") > the kdump kernel reuses the previous kernel's table without writing the > base register, so its newly allocated table is discarded when reuse > succeeds. If reuse fails, the previous patch programs the new table only > after disabling the IOMMU. Either way, the kdump kernel's table doesn't > need to be below 4G. > > The previous kernel still allocates its table from DMA32, so the kdump > kernel's check that the old table is below 4G still works. Drop > GFP_DMA32 for kdump kernels only. This one is OK as its regression fix. But with all the changes went in I am wondering why we need GFP_DMA32 flag for allocation. There are only two cases where transactions may be enabled: - During boot (BIOS enabled DMA) : kernel will disable IOMMU before configuring device table base address w/ new value - During kdump : We are reusing previous kernels device table. If it fails then kernel will disable IOMMU and re-configures it with new vlaue. Anyway, we will look into it in detail and run more experiments. For now this fix is fine. With that, Reviewed-by: Vasant Hegde -Vasant