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b=qPmVGwQFD7//hunr8IvV7F05BEjDm9zEIMA3iGjnFjHIZc+vFClbEwrtoqueBBkjW50uX0AP7ney+uhJqW9LMrAyYKdxFGQ/Z4IK0iauR1uMT9IT2JY5ML5EALN/xY+VF+UdLnGIXSBVo5ApcBPBOfWWOmbGNlFuY0JGDqopoXU= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from BL3PR12MB9049.namprd12.prod.outlook.com (2603:10b6:208:3b8::21) by CY5PR12MB6622.namprd12.prod.outlook.com (2603:10b6:930:42::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8922.39; Thu, 17 Jul 2025 06:55:22 +0000 Received: from BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad]) by BL3PR12MB9049.namprd12.prod.outlook.com ([fe80::ae6a:9bdd:af5b:e9ad%6]) with mapi id 15.20.8901.024; Thu, 17 Jul 2025 06:55:22 +0000 Message-ID: <145ab956-4dd5-4298-bbad-77759d70383f@amd.com> Date: Thu, 17 Jul 2025 01:55:18 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 4/4] iommu/amd: Fix host kdump support for SNP To: Vasant Hegde , joro@8bytes.org, suravee.suthikulpanit@amd.com, thomas.lendacky@amd.com, Sairaj.ArunKodilkar@amd.com, herbert@gondor.apana.org.au Cc: seanjc@google.com, pbonzini@redhat.com, will@kernel.org, robin.murphy@arm.com, john.allen@amd.com, davem@davemloft.net, bp@alien8.de, michael.roth@amd.com, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, kvm@vger.kernel.org References: <529c8436-1aeb-41bc-94bd-8b0f128e6222@amd.com> <49ef7e43-6a5d-452a-936b-87a573225d1e@amd.com> Content-Language: en-US From: "Kalra, Ashish" In-Reply-To: Content-Type: text/plain; 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If active SNP VMs are present, >>>> SNP_SHUTDOWN_EX fails as firmware checks all encryption-capable ASIDs >>>> to ensure none are in use and that a DF_FLUSH is not required. If a >>>> DF_FLUSH is required, the firmware returns DFFLUSH_REQUIRED, causing >>>> SNP_SHUTDOWN_EX to fail. >>>> >>>> This casues the kdump kernel to boot with IOMMU SNP enforcement still >>>> enabled and IOMMU completion wait buffers (CWBs), command buffers, >>>> device tables and event buffer registers remain locked and exclusive >>>> to the previous kernel. Attempts to allocate and use new buffers in >>>> the kdump kernel fail, as the hardware ignores writes to the locked >>>> MMIO registers (per AMD IOMMU spec Section 2.12.2.1). >>>> >>>> As a result, the kdump kernel cannot initialize the IOMMU or enable IRQ >>>> remapping which is required for proper operation. >>>> >>>> This results in repeated "Completion-Wait loop timed out" errors and a >>>> second kernel panic: "Kernel panic - not syncing: timer doesn't work >>>> through Interrupt-remapped IO-APIC" >>>> >>>> The following MMIO registers are locked and ignore writes after failed >>>> SNP shutdown: >>>> Device Table Base Address Register >>>> Command Buffer Base Address Register >>>> Event Buffer Base Address Register >>>> Completion Store Base Register/Exclusion Base Register >>>> Completion Store Limit Register/Exclusion Range Limit Register >>>> >>> >>> May be you can rephrase the description as first patch covered some of these >>> details >> >> We do need to include the complete description here as this is the final >> patch of the series which fixes the kdump boot. >> >> Do note, that the description in the first patch only mentions the >> IOMMU buffers - command, CWB and event buffers for reuse and this commit >> log covers all reusing and remapping required - IOMMU buffers, device table, >> etc. >> >>>> Instead of allocating new buffers, re-use the previous kernel’s pages >>>> for completion wait buffers, command buffers, event buffers and device >>>> tables and operate with the already enabled SNP configuration and >>>> existing data structures. >>>> >>>> This approach is now used for kdump boot regardless of whether SNP is >>>> enabled during kdump. >>>> >>>> The fix enables successful crashkernel/kdump operation on SNP hosts >>>> even when SNP_SHUTDOWN_EX fails. >>>> >>>> Fixes: c3b86e61b756 ("x86/cpufeatures: Enable/unmask SEV-SNP CPU feature") >>> >>> I am not sure why you have marked only this patch as Fixes? Also it won't fix >>> the kdump if someone just backports only this patch right? >>> >> >> As mentioned in the cover letter, this is the final patch of the series which >> actually fixes the SNP kdump boot, so i kept Fixes: tag as part of this patch. >>> I am not sure if i can add Fixes: tag to all the four patches in this series ? > > But just adding Fixes to this one patch is adding more confusion and > complicating backport process. > > Is this really a fix? Did kdump ever worked on SNP enabled system? If yes then > add Fixes to all patches. If not call it as an enhancement. > Well, kdump only worked on SNP enabled systems if there are no active SNP VMs. But i think it makes more sense to remove the Fixes: tag from these patch-series as this SNP kdump support is more or less a feature enhancement for SNP. Thanks, Ashish