Newer CPUs provide a second mechanism to detect operations with lock
prefix which go accross a cache line boundary. Such operations have to
take bus lock which causes a system wide performance degradation when
these operations happen frequently.
The new mechanism is not using the #AC exception. It triggers #DB and is
restricted to operations in user space. Kernel side split lock access can
only be detected by the #AC based variant. Contrary to the #AC based
mechanism the #DB based variant triggers _after_ the instruction was
executed. The mechanism is CPUID enumerated and contrary to the #AC
version which is based on the magic TEST_CTRL_MSR and model/family based
enumeration on the way to become architectural.
-----BEGIN PGP SIGNATURE-----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=lmi8
-----END PGP SIGNATURE-----
Merge tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull x86 bus lock detection updates from Thomas Gleixner:
"Support for enhanced split lock detection:
Newer CPUs provide a second mechanism to detect operations with lock
prefix which go accross a cache line boundary. Such operations have to
take bus lock which causes a system wide performance degradation when
these operations happen frequently.
The new mechanism is not using the #AC exception. It triggers #DB and
is restricted to operations in user space. Kernel side split lock
access can only be detected by the #AC based variant.
Contrary to the #AC based mechanism the #DB based variant triggers
_after_ the instruction was executed. The mechanism is CPUID
enumerated and contrary to the #AC version which is based on the magic
TEST_CTRL_MSR and model/family based enumeration on the way to become
architectural"
* tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
Documentation/admin-guide: Change doc for split_lock_detect parameter
x86/traps: Handle #DB for bus lock
x86/cpufeatures: Enumerate #DB for bus lock detection
64f8e73de0
