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Wed, 28 Feb 2024 11:48:01 -0800 From: To: , , , , , CC: , , , , , , , , , , , , , , Subject: [PATCH v1 1/1] vfio/nvgrace-gpu: Convey kvm that the device is wc safe Date: Wed, 28 Feb 2024 19:48:01 +0000 Message-ID: <20240228194801.2299-1-ankita@nvidia.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-NV-OnPremToCloud: ExternallySecured X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CO1PEPF000042AC:EE_|IA0PR12MB8225:EE_ X-MS-Office365-Filtering-Correlation-Id: c116ad6b-b969-421b-1e19-08dc38963336 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:216.228.118.232;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:mail.nvidia.com;PTR:dc7edge1.nvidia.com;CAT:NONE;SFS:(13230031)(36860700004)(82310400014);DIR:OUT;SFP:1101; X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 Feb 2024 19:48:13.4264 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c116ad6b-b969-421b-1e19-08dc38963336 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.118.232];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CO1PEPF000042AC.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR12MB8225 From: Ankit Agrawal The NVIDIA Grace Hopper GPUs have device memory that is supposed to be used as a regular RAM. It is accessible through CPU-GPU chip-to-chip cache coherent interconnect and is present in the system physical address space. The device memory is split into two regions - termed as usemem and resmem - in the system physical address space, with each region mapped and exposed to the VM as a separate fake device BAR [1]. Owing to a hardware defect for Multi-Instance GPU (MIG) feature [2], there is a requirement - as a workaround - for the resmem BAR to display uncached memory characteristics. Based on [3], on system with FWB enabled such as Grace Hopper, the requisite properties (uncached, unaligned access) can be achieved through a VM mapping (S1) of NORMAL_NC and host mapping (S2) of MT_S2_FWB_NORMAL_NC. KVM currently maps the MMIO region in S2 as MT_S2_FWB_DEVICE_nGnRE by default. The fake device BARs thus displays DEVICE_nGnRE behavior in the VM. The following table summarizes the behavior for the various S1 and S2 mapping combinations for systems with FWB enabled [3]. S1 | S2 | Result NORMAL_WB | NORMAL_NC | NORMAL_NC NORMAL_WT | NORMAL_NC | NORMAL_NC NORMAL_NC | NORMAL_NC | NORMAL_NC NORMAL_WB | DEVICE_nGnRE | DEVICE_nGnRE NORMAL_WT | DEVICE_nGnRE | DEVICE_nGnRE NORMAL_NC | DEVICE_nGnRE | DEVICE_nGnRE Recently a change was added that modifies this default behavior and make KVM map MMIO as MT_S2_FWB_NORMAL_NC when a VMA flag VM_ALLOW_ANY_UNCACHED is set. Setting S2 as MT_S2_FWB_NORMAL_NC provides the desired behavior (uncached, unaligned access) for resmem. Such setting is extended to the usemem as a middle-of-the-road setting to take it closer to the desired final system memory characteristics (cached, unaligned). This will eventually be fixed with the ongoing proposal [4]. To use VM_ALLOW_ANY_UNCACHED flag, the platform must guarantee that no action taken on the MMIO mapping can trigger an uncontained failure. The Grace Hopper satisfies this requirement. So set the VM_ALLOW_ANY_UNCACHED flag in the VMA. Applied over next-20240227. base-commit: 22ba90670a51 Link: https://lore.kernel.org/all/20240220115055.23546-4-ankita@nvidia.com/ [1] Link: https://www.nvidia.com/en-in/technologies/multi-instance-gpu/ [2] Link: https://developer.arm.com/documentation/ddi0487/latest/ section D8.5.5 [3] Link: https://lore.kernel.org/all/20230907181459.18145-2-ankita@nvidia.com/ [4] Cc: Alex Williamson Cc: Kevin Tian Cc: Jason Gunthorpe Cc: Vikram Sethi Cc: Zhi Wang Signed-off-by: Ankit Agrawal --- drivers/vfio/pci/nvgrace-gpu/main.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c index 25814006352d..5539c9057212 100644 --- a/drivers/vfio/pci/nvgrace-gpu/main.c +++ b/drivers/vfio/pci/nvgrace-gpu/main.c @@ -181,6 +181,24 @@ static int nvgrace_gpu_mmap(struct vfio_device *core_vdev, vma->vm_pgoff = start_pfn; + /* + * The VM_ALLOW_ANY_UNCACHED VMA flag is implemented for ARM64, + * allowing KVM stage 2 device mapping attributes to use Normal-NC + * rather than DEVICE_nGnRE, which allows guest mappings + * supporting write-combining attributes (WC). This also + * unlocks memory-like operations such as unaligned accesses. + * This setting suits the fake BARs as they are expected to + * demonstrate such properties within the guest. + * + * ARM does not architecturally guarantee this is safe, and indeed + * some MMIO regions like the GICv2 VCPU interface can trigger + * uncontained faults if Normal-NC is used. The nvgrace-gpu + * however is safe in that the platform guarantees that no + * action taken on the MMIO mapping can trigger an uncontained + * failure. Hence VM_ALLOW_ANY_UNCACHED is set in the VMA flags. + */ + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED); + return 0; }