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On platforms with a standalone media GT and media version 13.00 (MTL/ARL), memory shared between the CPU and the media GT's GuC must not be mapped cached on the CPU side: the CPU can otherwise read stale cache lines for data the GuC has already written. i915 implements this as Wa_22016122933 (see intel_gt_needs_wa_22016122933(), used by intel_guc_allocate_vma() and intel_gt_coherent_map_type()); xe never inherited it. The visible symptom on ARL is TLB invalidation acks stalling for a near-constant ~2.3s: the GuC writes the G2H ack in time, but the CPU keeps reading a stale (empty) view of the G2H CTB until the line is naturally evicted, so the fence timeout at 2.25s fires first. GuC log decode confirmed all invalidations were handled promptly by the firmware, and only the media GT was affected. See Link for the full investigation (three machines affected: 7d51, 7dd1, Arc Pro 130T). Making the mapping coherent instead of uncached does not help: with a GGTT PAT entry repurposed to WB|COH_2WAY the driver comes up and the media GT GuC runs, but the stalls remain (request-to-ack 2290ms). Uncached really is required here. Note that 2-way coherency is not normally reachable from a GGTT PTE (only 2 PAT bits), so that experiment needed a modified PAT table and may not reflect a supported configuration. Add the OOB workaround scoped like i915 (media version 13.00, media GT only - MEDIA_VERSION() OOB rules only match the media GT on standalone media platforms) and apply XE_BO_FLAG_NEEDS_UC to the GuC-shared allocations the CPU reads from: the CTBs, the GuC log, ADS, the SLPC shared data and the engine activity buffers. hwconfig and the G2G buffer are allocated on the primary GT only, where the workaround does not apply. Note that XE_BO_FLAG_NEEDS_UC drives both the CPU mapping (uncached) and the GGTT cache mode (XE_CACHE_NONE instead of XE_CACHE_WB), which is stricter than i915: i915 documents the workaround as WC on the CPU side and UC on the GPU side. A CPU-WC variant with the GGTT side kept at XE_CACHE_NONE was measured to be equally effective, but expressing it would require either a new BO flag or decoupling the GGTT cache-mode selection from XE_BO_FLAG_NEEDS_UC - simply swapping in XE_BO_FLAG_FORCE_WC silently relaxes the GPU side back to WB and the stalls return. The stricter mapping is kept here since it is the tested configuration and no throughput difference between the two was measurable; the extra plumbing can be added later if parity with i915 is preferred. Scope is limited to the GuC-shared allocations, which is where the failures were observed. i915 additionally covers media-GT LRC/ring state; extending xe to match can be done as a follow-up if wanted. Validation on two ARL machines (7d51 and 7dd1): before, 20-60 TLB invalidation ack stalls per day, every day, for weeks, on every kernel and on two GuC firmware versions (70.53.0 and 70.72.1). After: zero stalls in six weeks of combined runtime, over 10M TLB invalidations processed under the same workloads, across kernels 7.1.6, 7.1.8 and 7.2. The 7dd1 machine, which could not survive a day of media workloads on xe without a platform freeze, has been running xe full time since 11 August, including days with heavy video transcoding, with zero stalls and zero freezes. The coherency experiment, the CPU-WC measurements and the engine activity coverage gap were found by Navon John Lukose while A/B testing v2 on an ARL 7d51. Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8678 Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> Signed-off-by: Tales A. Mendonça <talesam@gmail.com> Tested-by: Navon John Lukose <navonjohnlukose@gmail.com> Reviewed-by: Matthew Brost <matthew.brost@intel.com> Signed-off-by: Matthew Brost <matthew.brost@intel.com> Link: https://patch.msgid.link/20260922144634.55130-4-talesam@gmail.com |
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer: Getting started with kernel development * Academic Researcher: Studying kernel internals and architecture * Security Expert: Hardening and vulnerability analysis * Backport/Maintenance Engineer: Maintaining stable kernels * System Administrator: Configuring and troubleshooting * Maintainer: Leading subsystems and reviewing patches * Hardware Vendor: Writing drivers for new hardware * Distribution Maintainer: Packaging kernels for distros * AI Coding Assistant: LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst