Intel disables hardware lock elision on all current CPUs, Hacker News

x 86 / msr: Add the IA 32 _ TSX_CTRL MSR

Transactional Synchronization Extensions (TSX) may be used on certain processors as part of a speculative side channel attack. A microcode update for existing processors that are vulnerable to this attack will add a new MSR – IA 32 _ TSX_CTRL to allow the system administrator the option to disable TSX as one of the possible mitigations. The CPUs which get this new MSR after a microcode upgrade are the ones which do not set MSR_IA 32 _ ARCH_CAPABILITIES.MDS_NO (bit 5) because those CPUs have CPUID.MD_CLEAR, i.e., the VERW implementation which clears all CPU buffers takes care of the TAA case as well.   [ Note that future processors that are not vulnerable will also support the IA32_TSX_CTRL MSR. ] Add defines for the new IA 32 _ TSX_CTRL MSR and its bits. TSX has two sub-features: 1. Restricted Transactional Memory (RTM) is an explicitly-used feature    where new instructions begin and end TSX transactions. 2. Hardware Lock Elision (HLE) is implicitly used when certain kinds of    “old” style locks are used by software. Bit 7 of the IA 32 _ ARCH_CAPABILITIES indicates the presence of the IA 32 _ TSX_CTRL MSR. There are two control bits in IA 32 _ TSX_CTRL MSR:   Bit 0: When set, it disables the Restricted Transactional Memory (RTM)          sub-feature of TSX (will force all transactions to abort on the XBEGIN instruction).   Bit 1: When set, it disables the enumeration of the RTM and HLE feature          (i.e. it will make CPUID (EAX=7) .EBX {bit4} and CPUID (EAX=7) .EBX {bit 11} read as 0). The other TSX sub-feature, Hardware Lock Elision (HLE), is unconditionally disabled by the new microcode but still enumerated as present by CPUID (EAX=7) .EBX {bit4}, unless disabled by IA 32 _ TSX_CTRL_MSR [1] – TSX_CTRL_CPUID_CLEAR. Signed-off-by: Pawan GuptaSigned-off-by: Borislav PetkovSigned-off-by: Thomas GleixnerTested-by: Neelima KrishnanReviewed-by: Mark GrossReviewed-by: Tony LuckReviewed-by: Josh Poimboeuf

1 files changed, 5 insertions, 0 deletions