linux-kernel - [PATCH v4 0/2] x86: Allow variable-sized event frame

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Message-ID: <20250512064353.1535984-1-xin@zytor.com>
Date: Sun, 11 May 2025 23:43:51 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        peterz@...radead.org, brgerst@...il.com
Subject: [PATCH v4 0/2] x86: Allow variable-sized event frame

This was initially posted as part of the FRED patch series and turned
down due to its unacceptable quality:
  https://lore.kernel.org/lkml/20230410081438.1750-31-xin3.li@intel.com/

Now comes another attempt to meet the bar.

A FRED event frame could contain different amount of information for
different event types, e.g., #MCE could push extra bytes of information,
or perhaps even for different instances of the same event type. Thus
the size of an event frame pushed by a FRED CPU is not fixed and the
address of a pt_regs structure that is used to save the user level
context of current task is not at a fixed offset from top of current
task kernel stack.

This patch set adds a new field named 'user_pt_regs' in the thread_info
structure to save the address of user level context pt_regs structure,
thus to eliminate the need of any advance information of event frame
size and allow a FRED CPU to push variable-sized event frame.

With the above change, we can remove the padding space at top of the
init stack because there is no user level context for init task.

Link to v3: https://lore.kernel.org/lkml/20250321053735.2479875-1-xin@zytor.com/

Change in v4:
* Drop the patch that zaps TOP_OF_KERNEL_STACK_PADDING on x86_64
  (Brian Gerst, hpa).

Change in v3:
* Replace "(struct pt_regs *)TOP_OF_INIT_STACK - 1" with
  (struct pt_regs *)__top_init_kernel_stack (Brian Gerst).
* Add declaration for __top_init_kernel_stack[] (Intel lkp).

Change in v2:
* Rebase on latest tip/master.

Xin Li (Intel) (2):
  x86/fred: Allow variable-sized event frame
  x86: Remove the padding space at top of the init stack

 arch/x86/entry/entry_fred.c        | 10 ++++++++++
 arch/x86/include/asm/processor.h   | 28 ++++++++++++++++++++--------
 arch/x86/include/asm/thread_info.h | 11 ++++++++---
 arch/x86/kernel/process.c          | 22 ++++++++++++++++++++++
 arch/x86/kernel/vmlinux.lds.S      |  7 +++++--
 include/linux/thread_info.h        |  1 +
 kernel/fork.c                      |  6 ++++++
 7 files changed, 72 insertions(+), 13 deletions(-)

base-commit: 8e3f385164626dc6bbf000decf04aa98e943e07e
-- 
2.49.0