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

[Hou Wenlong]

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@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Xin Li <xin3.li@xxxxxxxxx>
Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>

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(-)

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2.31.1