Best guess, since I can't find any code that actually uses them. Linux actively turns them off at boot...
r~ Richard Henderson (8): target-i386: Move breakpoint related functions to new file target-i386: Make check_hw_breakpoints static target-i386: Introduce cpu_x86_update_dr7 target-i386: Re-introduce optimal breakpoint removal target-i386: Move hw_*breakpoint_* functions target-i386: Optimize setting dr[0-3] target-i386: Handle I/O breakpoints target-i386: Check CR4[DE] for processing DR4/DR5 target-i386/Makefile.objs | 2 +- target-i386/bpt_helper.c | 324 ++++++++++++++++++++++++++++++++++++++++++++++ target-i386/cpu.h | 36 +----- target-i386/helper.c | 128 ------------------ target-i386/helper.h | 4 +- target-i386/machine.c | 8 +- target-i386/misc_helper.c | 34 ----- target-i386/seg_helper.c | 8 +- target-i386/translate.c | 30 ++++- 9 files changed, 365 insertions(+), 209 deletions(-) create mode 100644 target-i386/bpt_helper.c -- 2.1.0