Sohil reported seeing a split lock warning when running a test that
generates userspace #DB:

  x86/split lock detection: #DB: sigtrap_loop_64/4614 took a bus_lock trap at address: 0x4011ae


We investigated the issue and figured out:

  1) The warning is a false positive.

  2) It is not caused by the test itself.

  3) It occurs even when Bus Lock Detection (BLD) is disabled.

  4) It only happens on the first #DB on a CPU.


And the root cause is, at boot time, Linux zeros DR6.  This leads to
different DR6 values depending on whether the CPU supports BLD:

  1) On CPUs with BLD support, DR6 becomes 0xFFFF07F0 (bit 11, DR6.BLD,
     is cleared).

  2) On CPUs without BLD, DR6 becomes 0xFFFF0FF0.

Since only BLD-induced #DB exceptions clear DR6.BLD and other debug
exceptions leave it unchanged, even if the first #DB is unrelated to
BLD, DR6.BLD is still cleared.  As a result, such a first #DB is
misinterpreted as a BLD #DB, and a false warning is triggerred.


Fix the bug by initializing DR6 by writing its architectural reset
value at boot time.

DR7 suffers from a similar issue, apply the same fix.


This patch set is based on tip/x86/urgent branch as of today.

Link to the previous patch set v2:
https://lore.kernel.org/lkml/20250617073234.1020644-1-xin@zytor.com/

Changes in v3:
*) Polish initialize_debug_regs() (PeterZ).
*) Rewrite the comment for DR6_RESERVED definition (Sohil and Sean).
*) Reword the patch 2's changelog using Sean's description.
*) Explain the definition of DR7_FIXED_1 (Sohil).
*) Collect TB, RB, AB (PeterZ, Sohil and Sean).


Xin Li (Intel) (2):
  x86/traps: Initialize DR6 by writing its architectural reset value
  x86/traps: Initialize DR7 by writing its architectural reset value

 arch/x86/include/asm/debugreg.h      | 19 ++++++++++++----
 arch/x86/include/asm/kvm_host.h      |  2 +-
 arch/x86/include/uapi/asm/debugreg.h | 21 ++++++++++++++++-
 arch/x86/kernel/cpu/common.c         | 24 ++++++++------------
 arch/x86/kernel/kgdb.c               |  2 +-
 arch/x86/kernel/process_32.c         |  2 +-
 arch/x86/kernel/process_64.c         |  2 +-
 arch/x86/kernel/traps.c              | 34 +++++++++++++++++-----------
 arch/x86/kvm/x86.c                   |  4 ++--
 9 files changed, 72 insertions(+), 38 deletions(-)


base-commit: 2aebf5ee43bf0ed225a09a30cf515d9f2813b759
-- 
2.49.0