Re: [PATCH 01/21] arm64: entry: Defer setting of TPIDRRO_EL0 until exit to userspace

In preparation for using TPIDRRO_EL0 to point at 'current' while running
inside the kernel, defer its userspace initialisation from the
context-switch patch to exception return. This has the added benefit of
not having to worry about keeping the in-memory value and the register
value in-sync during preemptible sections.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/kernel/asm-offsets.c |  1 +
 arch/arm64/kernel/entry.S       |  4 +++-
 arch/arm64/kernel/process.c     | 16 +---------------
 arch/arm64/kernel/sys_compat.c  |  7 -------
 4 files changed, 5 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 9c853ed3ceab..6038ab3beb25 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c

@@ -42,6 +42,7 @@ int main(void)
 #endif
   BLANK();
   DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct task_struct, thread.cpu_context));
+  DEFINE(THREAD_TP_VALUE,	offsetof(struct task_struct, thread.uw.tp_value));
   DEFINE(THREAD_SCTLR_USER,	offsetof(struct task_struct, thread.sctlr_user));
 #ifdef CONFIG_ARM64_PTR_AUTH
   DEFINE(THREAD_KEYS_USER,	offsetof(struct task_struct, thread.keys_user));

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f63049ac32dc..59f045e496ec 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -47,7 +47,6 @@
 	b	.Lskip_tramp_vectors_cleanup\@
 	.if	\regsize == 64
 	mrs	x30, tpidrro_el0
-	msr	tpidrro_el0, xzr
 	.else
 	mov	x30, xzr
 	.endif

@@ -360,9 +359,12 @@ alternative_else_nop_endif
 	.if	\el == 0
 	ldr	x23, [sp, #S_SP]		// load return stack pointer
 	msr	sp_el0, x23
+	msr	tpidrro_el0, xzr
 	tst	x22, #PSR_MODE32_BIT		// native task?
 	b.eq	3f
 
+	ldr	x0, [tsk, #THREAD_TP_VALUE]
+	msr	tpidrro_el0, x0

 #ifdef CONFIG_ARM64_ERRATUM_845719
 alternative_if ARM64_WORKAROUND_845719
 #ifdef CONFIG_PID_IN_CONTEXTIDR

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 581f80e9b9b7..bfdc12166895 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c

@@ -256,17 +256,8 @@ static void tls_thread_flush(void)
 	if (system_supports_tpidr2())
 		write_sysreg_s(0, SYS_TPIDR2_EL0);
 
-	if (is_compat_task()) {
+	if (is_compat_task())
 		current->thread.uw.tp_value = 0;
-
-		/*
-		 * We need to ensure ordering between the shadow state and the
-		 * hardware state, so that we don't corrupt the hardware state
-		 * with a stale shadow state during context switch.
-		 */
-		barrier();
-		write_sysreg(0, tpidrro_el0);
-	}
 }
 
 static void flush_tagged_addr_state(void)

@@ -531,11 +522,6 @@ static void tls_thread_switch(struct task_struct *next)
 {
 	tls_preserve_current_state();
 
-	if (is_compat_thread(task_thread_info(next)))
-		write_sysreg(next->thread.uw.tp_value, tpidrro_el0);
-	else
-		write_sysreg(0, tpidrro_el0);
-
 	write_sysreg(*task_user_tls(next), tpidr_el0);
 	if (system_supports_tpidr2())
 		write_sysreg_s(next->thread.tpidr2_el0, SYS_TPIDR2_EL0);

diff --git a/arch/arm64/kernel/sys_compat.c b/arch/arm64/kernel/sys_compat.c
index 0451f96c2c3f..59eee301c6f4 100644
--- a/arch/arm64/kernel/sys_compat.c
+++ b/arch/arm64/kernel/sys_compat.c

@@ -89,13 +89,6 @@ long compat_arm_syscall(struct pt_regs *regs, int scno)
 
 	case __ARM_NR_compat_set_tls:
 		current->thread.uw.tp_value = regs->regs[0];
-
-		/*
-		 * Protect against register corruption from context switch.
-		 * See comment in tls_thread_flush.
-		 */
-		barrier();
-		write_sysreg(regs->regs[0], tpidrro_el0);
 		return 0;
 
 	default: