arm64: Move handling of erratum 1418040 into C code

Instead of dealing with erratum 1418040 on each entry and exit,
let's move the handling to __switch_to() instead, which has
several advantages:

- It can be applied when it matters (switching between 32 and 64
  bit tasks).
- It is written in C (yay!)
- It can rely on static keys rather than alternatives

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200731173824.107480-2-maz@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -170,19 +170,6 @@ alternative_cb_end
 	stp	x28, x29, [sp, #16 * 14]
 
 	.if	\el == 0
-	.if	\regsize == 32
-	/*
-	 * If we're returning from a 32-bit task on a system affected by
-	 * 1418040 then re-enable userspace access to the virtual counter.
-	 */
-#ifdef CONFIG_ARM64_ERRATUM_1418040
-alternative_if ARM64_WORKAROUND_1418040
-	mrs	x0, cntkctl_el1
-	orr	x0, x0, #2	// ARCH_TIMER_USR_VCT_ACCESS_EN
-	msr	cntkctl_el1, x0
-alternative_else_nop_endif
-#endif
-	.endif
 	clear_gp_regs
 	mrs	x21, sp_el0
 	ldr_this_cpu	tsk, __entry_task, x20
@@ -294,14 +281,6 @@ alternative_else_nop_endif
 	tst	x22, #PSR_MODE32_BIT		// native task?
 	b.eq	3f
 
-#ifdef CONFIG_ARM64_ERRATUM_1418040
-alternative_if ARM64_WORKAROUND_1418040
-	mrs	x0, cntkctl_el1
-	bic	x0, x0, #2			// ARCH_TIMER_USR_VCT_ACCESS_EN
-	msr	cntkctl_el1, x0
-alternative_else_nop_endif
-#endif
-
 #ifdef CONFIG_ARM64_ERRATUM_845719
 alternative_if ARM64_WORKAROUND_845719
 #ifdef CONFIG_PID_IN_CONTEXTIDR

arch/arm64/kernel/process.c

@@ -515,6 +515,39 @@ static void entry_task_switch(struct task_struct *next)
 	__this_cpu_write(__entry_task, next);
 }
 
+/*
+ * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT.
+ * Assuming the virtual counter is enabled at the beginning of times:
+ *
+ * - disable access when switching from a 64bit task to a 32bit task
+ * - enable access when switching from a 32bit task to a 64bit task
+ */
+static void erratum_1418040_thread_switch(struct task_struct *prev,
+					  struct task_struct *next)
+{
+	bool prev32, next32;
+	u64 val;
+
+	if (!(IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) &&
+	      cpus_have_const_cap(ARM64_WORKAROUND_1418040)))
+		return;
+
+	prev32 = is_compat_thread(task_thread_info(prev));
+	next32 = is_compat_thread(task_thread_info(next));
+
+	if (prev32 == next32)
+		return;
+
+	val = read_sysreg(cntkctl_el1);
+
+	if (!next32)
+		val |= ARCH_TIMER_USR_VCT_ACCESS_EN;
+	else
+		val &= ~ARCH_TIMER_USR_VCT_ACCESS_EN;
+
+	write_sysreg(val, cntkctl_el1);
+}
+
 /*
  * Thread switching.
  */
@@ -530,6 +563,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 	entry_task_switch(next);
 	uao_thread_switch(next);
 	ssbs_thread_switch(next);
+	erratum_1418040_thread_switch(prev, next);
 
 	/*
 	 * Complete any pending TLB or cache maintenance on this CPU in case