[PATCH v2] arm64: Preserve the user r/w register tpidr_el0 on context switch and fork in compat mode

[nerv@dawncrow.de (André Hentschel)]

From: Andr? Hentschel <nerv@dawncrow.de>

Since commit a4780adeefd042482f624f5e0d577bf9cdcbb760 the user writeable TLS
register on ARM is preserved per thread.

This patch does it analogous to the ARM patch, but for compat mode on ARM64.

Signed-off-by: Andr? Hentschel <nerv@dawncrow.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>

---
This patch is against Linux 4.1-rc1 (b787f68c36d49bb1d9236f403813641efa74a031)

v2: Trying to address suggestions by Will Deacon

@Will Deacon: 	The macro you suggested is not helpful it seems, instead i introduced
		static functions. They also could be macros i guess, but it seems
		much cleaner to me to use functions as it only affects one file.
Should the final version be send to rmk's patchtracker or will someone else pick it up?


 arch/arm64/include/asm/processor.h |  3 +++
 arch/arm64/kernel/process.c        | 41 +++++++++++++++++++++++++++++---------
 2 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index d2c37a1..466a851 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -79,6 +79,9 @@ struct cpu_context {
 struct thread_struct {
 	struct cpu_context	cpu_context;	/* cpu context */
 	unsigned long		tp_value;
+#ifdef CONFIG_COMPAT
+	unsigned long		tp_compat;
+#endif
 	struct fpsimd_state	fpsimd_state;
 	unsigned long		fault_address;	/* fault info */
 	unsigned long		fault_code;	/* ESR_EL1 value */
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index c6b1f3b..630f44b 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -58,6 +58,22 @@ unsigned long __stack_chk_guard __read_mostly;
 EXPORT_SYMBOL(__stack_chk_guard);
 #endif
 
+static unsigned long get_tp_compat(struct thread_struct *p)
+{
+#ifdef CONFIG_COMPAT
+	return p->tp_compat;
+#else
+	return 0;
+#endif
+}
+
+static void set_tp_compat(struct thread_struct *p, unsigned long tpidr)
+{
+#ifdef CONFIG_COMPAT
+	p->tp_compat = tpidr;
+#endif
+}
+
 void soft_restart(unsigned long addr)
 {
 	setup_mm_for_reboot();
@@ -219,6 +235,7 @@ static void tls_thread_flush(void)
 
 	if (is_compat_task()) {
 		current->thread.tp_value = 0;
+		set_tp_compat(&current->thread, 0);
 
 		/*
 		 * We need to ensure ordering between the shadow state and the
@@ -259,17 +276,22 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 	memset(&p->thread.cpu_context, 0, sizeof(struct cpu_context));
 
 	if (likely(!(p->flags & PF_KTHREAD))) {
+		unsigned long tpidr;
 		*childregs = *current_pt_regs();
 		childregs->regs[0] = 0;
+		/*
+		 * Read the current TLS pointer from tpidr_el0 as it may be
+		 * out-of-sync with the saved value.
+		 */
+		asm("mrs %0, tpidr_el0" : "=r" (tpidr));
 		if (is_compat_thread(task_thread_info(p))) {
+			set_tp_compat(&p->thread, tpidr);
+
 			if (stack_start)
 				childregs->compat_sp = stack_start;
 		} else {
-			/*
-			 * Read the current TLS pointer from tpidr_el0 as it may be
-			 * out-of-sync with the saved value.
-			 */
-			asm("mrs %0, tpidr_el0" : "=r" (tls));
+			tls = tpidr;
+
 			if (stack_start) {
 				/* 16-byte aligned stack mandatory on AArch64 */
 				if (stack_start & 15)
@@ -302,13 +324,14 @@ static void tls_thread_switch(struct task_struct *next)
 {
 	unsigned long tpidr, tpidrro;
 
-	if (!is_compat_task()) {
-		asm("mrs %0, tpidr_el0" : "=r" (tpidr));
+	asm("mrs %0, tpidr_el0" : "=r" (tpidr));
+	if (is_compat_task())
+		set_tp_compat(&current->thread, tpidr);
+	else
 		current->thread.tp_value = tpidr;
-	}
 
 	if (is_compat_thread(task_thread_info(next))) {
-		tpidr = 0;
+		tpidr = get_tp_compat(&next->thread);
 		tpidrro = next->thread.tp_value;
 	} else {
 		tpidr = next->thread.tp_value;