x86/process: Correct and optimize TIF_BLOCKSTEP switch

The debug control MSR is "highly magical" as the blockstep bit can be
cleared by hardware under not well documented circumstances.

So a task switch relying on the bit set by the previous task (according to
the previous tasks thread flags) can trip over this and not update the flag
for the next task.

To fix this its required to handle DEBUGCTLMSR_BTF when either the previous
or the next or both tasks have the TIF_BLOCKSTEP flag set.

While at it avoid branching within the TIF_BLOCKSTEP case and evaluating
boot_cpu_data twice in kernels without CONFIG_X86_DEBUGCTLMSR.

x86_64: arch/x86/kernel/process.o
text	data	bss	dec	 hex
3024    8577    16      11617    2d61	Before
3008	8577	16	11601	 2d51	After

i386: No change

[ tglx: Made the shift value explicit, use a local variable to make the
code readable and massaged changelog]

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/20170214081104.9244-3-khuey@kylehuey.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index d8b5f8ab8ef9e79fb76586bc3ca0f7f27325123e..4c928f332f8f8139320129ba7f17b5f68eaf2a77 100644 (file)
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -127,6 +127,7 @@
 
 /* DEBUGCTLMSR bits (others vary by model): */
 #define DEBUGCTLMSR_LBR                        (1UL <<  0) /* last branch recording */
+#define DEBUGCTLMSR_BTF_SHIFT          1
 #define DEBUGCTLMSR_BTF                        (1UL <<  1) /* single-step on branches */
 #define DEBUGCTLMSR_TR                 (1UL <<  6)
 #define DEBUGCTLMSR_BTS                        (1UL <<  7)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index ea9ea2582dab6cedb867739e3bc44f1f70a019bc..83fa3cb4f8f08bac04717325802858e6f5bb07e4 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -222,13 +222,15 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
 
        propagate_user_return_notify(prev_p, next_p);
 
-       if ((tifp ^ tifn) & _TIF_BLOCKSTEP) {
-               unsigned long debugctl = get_debugctlmsr();
+       if ((tifp & _TIF_BLOCKSTEP || tifn & _TIF_BLOCKSTEP) &&
+           arch_has_block_step()) {
+               unsigned long debugctl, msk;
 
+               rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
                debugctl &= ~DEBUGCTLMSR_BTF;
-               if (tifn & _TIF_BLOCKSTEP)
-                       debugctl |= DEBUGCTLMSR_BTF;
-               update_debugctlmsr(debugctl);
+               msk = tifn & _TIF_BLOCKSTEP;
+               debugctl |= (msk >> TIF_BLOCKSTEP) << DEBUGCTLMSR_BTF_SHIFT;
+               wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
        }
 
        if ((tifp ^ tifn) & _TIF_NOTSC) {