[RFC PATCH 03/13] x86/hw_breakpoint: Add arch_reinstall_hw_breakpoint
From: Jinchao Wang <hidden>
Date: 2026-07-14 18:23:34
Also in:
linux-doc, linux-mm, linux-perf-users, lkml
Subsystem:
the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
The new arch_reinstall_hw_breakpoint() function can be used in an atomic context, unlike the more expensive free and re-allocation path. This allows callers to efficiently re-establish an existing breakpoint, and x86 advertises the capability via HAVE_REINSTALL_HW_BREAKPOINT. Since a REINSTALL may change bp_len, setup_hwbp() must clear the slot's stale len/type and enable bits in DR7 before re-encoding: OR-merging the new encoding over the old one would keep the CPU watching with the stale width (verified in QEMU by reading DR7 after re-arming watch_len=1 over a len8 breakpoint: 0x999906aa merged without the clearing, 0x199906aa with it). Signed-off-by: Jinchao Wang <redacted> --- arch/x86/Kconfig | 1 + arch/x86/include/asm/hw_breakpoint.h | 2 ++ arch/x86/kernel/hw_breakpoint.c | 16 ++++++++++++++-- 3 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bdad90f210e4..5be698db0241 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig@@ -246,6 +246,7 @@ config X86 select HAVE_FUNCTION_TRACER select HAVE_GCC_PLUGINS select HAVE_HW_BREAKPOINT + select HAVE_REINSTALL_HW_BREAKPOINT select HAVE_IOREMAP_PROT select HAVE_IRQ_EXIT_ON_IRQ_STACK if X86_64 select HAVE_IRQ_TIME_ACCOUNTING
diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
index aa6adac6c3a2..c22cc4e87fc5 100644
--- a/arch/x86/include/asm/hw_breakpoint.h
+++ b/arch/x86/include/asm/hw_breakpoint.h@@ -21,6 +21,7 @@ struct arch_hw_breakpoint { enum bp_slot_action { BP_SLOT_ACTION_INSTALL, + BP_SLOT_ACTION_REINSTALL, BP_SLOT_ACTION_UNINSTALL, };
@@ -65,6 +66,7 @@ extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused, int arch_install_hw_breakpoint(struct perf_event *bp); +int arch_reinstall_hw_breakpoint(struct perf_event *bp); void arch_uninstall_hw_breakpoint(struct perf_event *bp); void hw_breakpoint_pmu_read(struct perf_event *bp); void hw_breakpoint_pmu_unthrottle(struct perf_event *bp);
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 877509539300..4221dbb899f9 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c@@ -100,6 +100,10 @@ static int manage_bp_slot(struct perf_event *bp, enum bp_slot_action action) old_bp = NULL; new_bp = bp; break; + case BP_SLOT_ACTION_REINSTALL: + old_bp = bp; + new_bp = bp; + break; case BP_SLOT_ACTION_UNINSTALL: old_bp = bp; new_bp = NULL;
@@ -134,10 +138,13 @@ static void setup_hwbp(struct arch_hw_breakpoint *info, int slot, bool enable) __this_cpu_write(cpu_debugreg[slot], info->address); dr7 = this_cpu_read(cpu_dr7); + /* + * Clear the slot's stale len/type and enable bits first: a REINSTALL + * with a different bp_len would otherwise OR-merge both encodings. + */ + dr7 &= ~__encode_dr7(slot, 0xf, 0); if (enable) dr7 |= encode_dr7(slot, info->len, info->type); - else - dr7 &= ~__encode_dr7(slot, info->len, info->type); /* * Enabling:
@@ -188,6 +195,11 @@ int arch_install_hw_breakpoint(struct perf_event *bp) return arch_manage_bp(bp, BP_SLOT_ACTION_INSTALL); } +int arch_reinstall_hw_breakpoint(struct perf_event *bp) +{ + return arch_manage_bp(bp, BP_SLOT_ACTION_REINSTALL); +} + void arch_uninstall_hw_breakpoint(struct perf_event *bp) { arch_manage_bp(bp, BP_SLOT_ACTION_UNINSTALL);
--
2.53.0