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