x86/debug: Sync BTF earlier

Move the BTF sync near the DR6 load, as this will be the only common
code guaranteed to run on every #DB.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200902133200.786888252@infradead.org

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 81a2fb7..9945642 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -749,6 +749,13 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
        /* Filter out all the reserved bits which are preset to 1 */
        dr6 &= ~DR6_RESERVED;
 
+       /*
+        * The SDM says "The processor clears the BTF flag when it
+        * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
+        * TIF_BLOCKSTEP in sync with the hardware BTF flag.
+        */
+       clear_thread_flag(TIF_BLOCKSTEP);
+
        return dr6;
 }
 
@@ -783,13 +790,6 @@ static void handle_debug(struct pt_regs *regs, unsigned long dr6, bool user)
        int si_code;
 
        /*
-        * The SDM says "The processor clears the BTF flag when it
-        * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
-        * TIF_BLOCKSTEP in sync with the hardware BTF flag.
-        */
-       clear_thread_flag(TIF_BLOCKSTEP);
-
-       /*
         * If DR6 is zero, no point in trying to handle it. The kernel is
         * not using INT1.
         */