[RFC PATCH v2 03/13] x86/hw_breakpoint: Add arch_reinstall_hw_breakpoint

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 <[email protected]>
---
 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 76886467708b..e2dbd43d8e39 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:
@@ -198,6 +205,11 @@ 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