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. We apply the same fix.
This patch set is based on tip/x86/urgent branch as of today.
Changes in v2:
*) Use debug register indexes rather than DR_* macros (PeterZ and Sean).
*) Use DR7_FIXED_1 as the architectural reset value of DR7 (Sean).
*) Move the DR6 fix patch to the first of the patch set to ease backporting.
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 | 14 ++++++++----
arch/x86/include/asm/kvm_host.h | 2 +-
arch/x86/include/uapi/asm/debugreg.h | 7 +++++-
arch/x86/kernel/cpu/common.c | 17 ++++++--------
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, 50 insertions(+), 34 deletions(-)
base-commit: 594902c986e269660302f09df9ec4bf1cf017b77
--
2.49.0