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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----- iQJHBAABCgAxFiEEQp8+kY+LLUocC4bMphj1TA10mKEFAmCGkr8THHRnbHhAbGlu dXRyb25peC5kZQAKCRCmGPVMDXSYodUKD/9tUXhInR7+1ykEHpMvdmSp48vqY3nc sKmT22pPl+OchnJ62mw3T8gKpBYVleJmcCaY2qVx7hfaVcWApLGJvX4tmfXmv422 XDSJ6b8Os6wfgx5FR//I17z8ZtXnnuKkPrTMoRsQUw2qLq31y6fdQv+GW/cc1Kpw mengjmPE+HnpaKbtuQfPdc4a+UvLjvzBMAlDZPTBPKYrP4FFqYVnUVwyTg5aLVDY gHz4V8+b502RS/zPfTAtE3J848od+NmcUPdFlcG9DVA+hR0Rl0thvruCTFiD2vVh i9DJ7INof5FoJDEzh0dGsD7x+MB6OY8GZyHdUMeGgIRPtWkqrG52feQQIn2YYlaL fB3DlpNv7NIJ/0JMlALvh8S0tEoOcYdHqH+M/3K/zbzecg/FAo+lVo8WciGLPqWs ykUG5/f/OnlTvgB8po1ebJu0h0jHnoK9heWWXk9zWIRVDPXHFOWKW3kSbTTb3icR 9hfjP/SNejpmt9Ju1OTwsgnV7NALIdVX+G5jyIEsjFl31Co1RZNYhHLFvi11FWlQ /ssvFK9O5ZkliocGCAN9+yuOnM26VqWSCE4fis6/2aSgD2Y4Gpvb//cP96SrcNAH u8eXNvGLlniJP3F3JImWIfIPQTrpvQhcU4eZ6NtviXqj/utQXX6c9PZ1PLYpcvUh 9AWF8rwhT8X4oA== =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 |
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