x86,kgdb: Implement early hardware breakpoint debugging

It is not possible to use the hw_breakpoint.c API prior to mm_init(),
but it is possible to use hardware breakpoints with the kernel
debugger.

Prior to smp_init() it is possible to simply write to the dr registers
of the boot cpu directly.  This can be used up until the
kgdb_arch_late() is invoked, at which point the standard hw_breakpoint.c
API will get used.

CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

1 files changed, 28 insertions, 2 deletions

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index 2b71ec41869f..4f4af75b9482 100644
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -199,6 +199,8 @@ static struct hw_breakpoint {
         struct perf_event       **pev;
 } breakinfo[4];
 
+static unsigned long early_dr7;
+
 static void kgdb_correct_hw_break(void)
 {
         int breakno;
@@ -210,6 +212,14 @@ static void kgdb_correct_hw_break(void)
                 int cpu = raw_smp_processor_id();
                 if (!breakinfo[breakno].enabled)
                         continue;
+                if (dbg_is_early) {
+                        set_debugreg(breakinfo[breakno].addr, breakno);
+                        early_dr7 |= encode_dr7(breakno,
+                                                breakinfo[breakno].len,
+                                                breakinfo[breakno].type);
+                        set_debugreg(early_dr7, 7);
+                        continue;
+                }
                 bp = *per_cpu_ptr(breakinfo[breakno].pev, cpu);
                 info = counter_arch_bp(bp);
                 if (bp->attr.disabled != 1)
@@ -224,7 +234,8 @@ static void kgdb_correct_hw_break(void)
                 if (!val)
                         bp->attr.disabled = 0;
         }
-        hw_breakpoint_restore();
+        if (!dbg_is_early)
+                hw_breakpoint_restore();
 }
 
 static int hw_break_reserve_slot(int breakno)
@@ -233,6 +244,9 @@ static int hw_break_reserve_slot(int breakno)
         int cnt = 0;
         struct perf_event **pevent;
 
+        if (dbg_is_early)
+                return 0;
+
         for_each_online_cpu(cpu) {
                 cnt++;
                 pevent = per_cpu_ptr(breakinfo[breakno].pev, cpu);
@@ -258,6 +272,9 @@ static int hw_break_release_slot(int breakno)
         struct perf_event **pevent;
         int cpu;
 
+        if (dbg_is_early)
+                return 0;
+
         for_each_online_cpu(cpu) {
                 pevent = per_cpu_ptr(breakinfo[breakno].pev, cpu);
                 if (dbg_release_bp_slot(*pevent))
@@ -302,7 +319,11 @@ static void kgdb_remove_all_hw_break(void)
                 bp = *per_cpu_ptr(breakinfo[i].pev, cpu);
                 if (bp->attr.disabled == 1)
                         continue;
-                arch_uninstall_hw_breakpoint(bp);
+                if (dbg_is_early)
+                        early_dr7 &= ~encode_dr7(i, breakinfo[i].len,
+                                                 breakinfo[i].type);
+                else
+                        arch_uninstall_hw_breakpoint(bp);
                 bp->attr.disabled = 1;
         }
 }
@@ -379,6 +400,11 @@ void kgdb_disable_hw_debug(struct pt_regs *regs)
         for (i = 0; i < 4; i++) {
                 if (!breakinfo[i].enabled)
                         continue;
+                if (dbg_is_early) {
+                        early_dr7 &= ~encode_dr7(i, breakinfo[i].len,
+                                                 breakinfo[i].type);
+                        continue;
+                }
                 bp = *per_cpu_ptr(breakinfo[i].pev, cpu);
                 if (bp->attr.disabled == 1)
                         continue;