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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?wD4iRyI0wivVdjUJIaEn+Oh6prgAuLd3qMsY4SECwSZ8Y+RRkFP+0aw+OYjK?= =?us-ascii?Q?w3TNJzwgQC0R6OqoJsSC+8LtDVrFIQL8f+CWO+hJnIOt69r2W9wrGaZ8WM2Q?= =?us-ascii?Q?AAwLJiO3K6x1npM/5tsEH3pk5hMVIGkjeqjf8JCsclUjex4zRWPDa3JvTEKN?= =?us-ascii?Q?rbMDwWMEEjirAgBKSFnb0Y1pioOPnxqUeyhTItlX6/hrcufkT8zF3MVmOqYW?= =?us-ascii?Q?iT1IPurP/PCuNF1cI9ojs6bdGIeJumqqPtgXHpuehsIQ0J9opkOd+QQpkyy7?= =?us-ascii?Q?kNpH/cmwrmIdYOxJMFS97GCFtTze0HU5n53f66MVNYs5BhT12HbRO1BxCYpU?= =?us-ascii?Q?JvZ2saI+mHf9qWiplRtW5UZYClFzcUaEO/SwDg+UZKVOMKAfECKRkyCW7/Rq?= =?us-ascii?Q?dqcFuq5GlD3X+0cZCHIv5zDMeUPlUBjuQGD7qnB94ju9C/siKnOKHHdPxfu7?= =?us-ascii?Q?aDq8ZrT5tdek/oqkHr1rzupIQDQwCaM5mvU/vQ1IllP1Pz5sWYGp6vwp1daF?= =?us-ascii?Q?zp7l//OeZnsmieg3Cg6qL56B983kHrktcuKCvubjF/ulZrHTy3Ro8R7Yf30D?= =?us-ascii?Q?Atla+4g3NNjriZQxYIbcDqQV6rNKfCBAWbPAhcere2vnrTa8e14u/ftJA2wW?= =?us-ascii?Q?0Udm05fsxQmNi5qM1sxdMEAQJAbfs6UyMRJ7bupPUQzkNAPFMsuNw79tIb3B?= =?us-ascii?Q?slVnPuKcPRQYSYFZXvC1iEe0Xk4bIqg9JoImq6J8FFcBQz8O00y7Ok+lA1uR?= =?us-ascii?Q?2sIjpnAI4Ao3Pal6zM41IEXiWroWe1OFjisbYFLnPbuI5SMlLZlgpHbWnyQQ?= =?us-ascii?Q?X6VJDg4YYRYbjYNeOA+G+oyN38deSxF+UPwlwy2v3IRXvFvSAjmuqQFLJz4w?= =?us-ascii?Q?1VqmUFCycVrFi8rxZGQjKnCHS5p/xyZV+FlM/kQQzTcZzxihseYFcsWq1661?= =?us-ascii?Q?9SyPipI5vchX3kYPeHY5bRtgXsts32h1E0SVwGc+aJpA4Imac3wzfytindDJ?= =?us-ascii?Q?K6r4el5dj1rh9eTbFE2fNAroNJUa5euNbbLDBEigVRubyTushOOl6edcFhUK?= =?us-ascii?Q?mYCXyZYtccklJqb3sn1sqb5JzLdXy/qJl9LHnZf69aNkENxkKo5qS8hbZGkO?= =?us-ascii?Q?tLN2NnbYta0w5JZlGhfD4yxFsFYIWSVaxcJk+DEzVFsboCaoI5TUT2550FZ/?= =?us-ascii?Q?g22wLZV9thZkh6a/YZ7MhS5gruKyA1EqPnjFQZa/frTAaEH2faKvWUFkkHjt?= =?us-ascii?Q?XFWvqPK8MVWYh3y1oZrNTpcfpQR8wa8bdddYTe5P+nx+j0ZdDFxmeRX2GY4z?= =?us-ascii?Q?rbKNU2mG8jMk3ouMUjdTMqUSU9y2ito8iC3m1onKupXW+Pxrs68eytKcZ94+?= =?us-ascii?Q?tn2yubaPxMVGptAwYgjDdiudlJ6oUiuGbDZWwgwADK0I9tduRCNr1AdzbPUI?= =?us-ascii?Q?Mp8iBOf4gB4YTv6bBCApMb6UdCdH0cWhmXJ9gkq73L8aCgV/FT01qqvtxWLi?= =?us-ascii?Q?VUPbbrWkYWdqKp61J8grEjBh+mE3dkmlet1EEZVkhUFVey8WL3dYjh93p4lF?= =?us-ascii?Q?/OIJIa2ce+UVq46yI1mMty7F1NIm6kdR7TU5qrPhyiEFFd1RhIziDXSaumWF?= =?us-ascii?Q?R04bp41I6lSutAxQBJmmmgZra0/Fw5Y4t0/TA3yW8mdXUbEu4XnekU1zpPId?= =?us-ascii?Q?2SOxUIJG+cvgLsVqwRZsiOB8vHYzaUeHiIDWOlSSrj+Ye2nKDVZC1UcbIsjb?= =?us-ascii?Q?qFYt+YvdjQ=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 944eef36-93f0-4559-606e-08de5db337f8 X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Jan 2026 14:49:05.4424 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: ZOOrRPOAE3YAo2hHxfnnUYbcockIkO0yXuSA4rqfC6Yba2EaFbsmITTSCZakAB4f X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA3PR12MB7975 On Tue, Jan 27, 2026 at 01:50:54PM +0000, Will Deacon wrote: > Argh, I had conflated a transaction using SSID 0 vs a transaction > without a substream at all. So I think this makes sense now... Yeah, it is bit subtle, but as a SW choice the iommu subsystem reserves PASID 0/SSID 0 as the "untagged" translation. Several HW's force this in their implementation (ie AMD) ARM however includes a "Substream Valid" in the input bus. Linux doesn't use the combination "Substream Valid, SSID=0", that should never occur. If it wrongly does happen then IDENTITY will generate a fault, either C_BAD_CD (due to it being non-valid) or C_BAD_SUBSTREAMID (due to S1CDMax disabling substreams). While PAGING will either fault with C_BAD_SUBSTREAMID (S2 paging domain) or success when S1DSS=b10. Jason