linux-kernel - [PATCH 0/2] x86/fred: Fix two problems during the FRED initialization

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Date: Fri, 21 Jun 2024 21:12:00 +0800
From: "Hou Wenlong" <houwenlong.hwl@...group.com>
To: linux-kernel@...r.kernel.org
Cc: "Lai Jiangshan" <jiangshan.ljs@...group.com>,
  "Hou Wenlong" <houwenlong.hwl@...group.com>,
  "Thomas Gleixner" <tglx@...utronix.de>,
  "Ingo Molnar" <mingo@...hat.com>,
  "Borislav Petkov" <bp@...en8.de>,
  "Dave Hansen" <dave.hansen@...ux.intel.com>,
   <x86@...nel.org>,
  "H. Peter Anvin" <hpa@...or.com>,
  "Xin Li" <xin3.li@...el.com>,
  "Jacob Pan" <jacob.jun.pan@...ux.intel.com>,
  "Rick Edgecombe" <rick.p.edgecombe@...el.com>,
  "Paolo Bonzini" <pbonzini@...hat.com>
Subject: [PATCH 0/2] x86/fred: Fix two problems during the FRED initialization

When I reviewed the FRED code and attempted to implement a FRED-like
event delivery for my PV guest, I encountered two problems which I may
have misunderstood.

One issue is that FRED can be disabled in trap_init(), but
sysvec_install() can be called before trap_init(), thus the system
interrupt handler is not installed into the IDT if FRED is disabled
later. Initially, I attempted to parse the cmdline and decide whether to
enable or disable FRED after parse_early_param(). However, I ultimately
chose to always install the system handler into the IDT in
sysvec_install(), which is simple and should be sufficient.

Another problem is that the page fault handler (exc_page_fault()) is
installed into the IDT before FRED is enabled. Consequently, if a #PF is
triggered in this gap, the handler would receive the wrong CR2 from the
stack if FRED feature is present. To address this, I added a page fault
entry stub for FRED similar to the debug entry. However, I'm uncertain
whether this is enough reason to add a new entry. Perhaps a static key
may suffice to indicate whether FRED setup is completed and the handler
can use it.

Note: I didn't test them on the Intel emulator, as I'm not familiar with
configuring it to boot with my compiled kernel.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Xin Li <xin3.li@...el.com>
Cc: Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>

Hou Wenlong (2):
  x86/fred: Always install system interrupt handler into IDT
  x86/fred: Add a page fault entry stub for FRED

 arch/x86/entry/entry_fred.c     |  2 +-
 arch/x86/include/asm/idtentry.h | 15 +++++++++++++--
 arch/x86/mm/fault.c             | 19 +++++++++++++++----
 3 files changed, 29 insertions(+), 7 deletions(-)

--
2.31.1