[PATCH v4 04/21] arm64: Mask all exceptions during kernel_exit

James Morse
2017-10-19T14:59:02Z

To take RAS Exceptions as quickly as possible we need to keep SError
unmasked as much as possible. We need to mask it during kernel_exit
as taking an error from this code will overwrite the exception-registers.

Adding a naked 'disable_daif' to kernel_exit causes a performance problem
for micro-benchmarks that do no real work, (e.g. calling getpid() in a
loop). This is because the ret_to_user loop has already masked IRQs so
that the TIF_WORK_MASK thread flags can't change underneath it, adding
disable_daif is an additional self-synchronising operation.

In the future, the RAS APEI code may need to modify the TIF_WORK_MASK
flags from an SError, in which case the ret_to_user loop must mask SError
while it examines the flags.

Disable all exceptions for return to EL1. For return to EL0 get the
ret_to_user loop to leave all exceptions masked once it has done its
work, this avoids an extra pstate-write.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

---
Changes since v3:
* swapped verb/daif word-order.

arch/arm64/kernel/entry.S | 10 +++++-----
arch/arm64/kernel/signal.c | 8 ++++++--
2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index e1c59d4008a8..f7d7bf9d76e7 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -221,6 +221,8 @@ alternative_else_nop_endif

.macro kernel_exit, el
.if \el != 0
+ disable_daif
+
/* Restore the task's original addr_limit. */
ldr x20, [sp, #S_ORIG_ADDR_LIMIT]
str x20, [tsk, #TSK_TI_ADDR_LIMIT]
@@ -517,8 +519,6 @@ el1_da:
mov x2, sp // struct pt_regs
bl do_mem_abort

- // disable interrupts before pulling preserved data off the stack
- disable_irq
kernel_exit 1
el1_sp_pc:
/*
@@ -793,7 +793,7 @@ ENDPROC(el0_irq)
* and this includes saving x0 back into the kernel stack.
*/
ret_fast_syscall:
- disable_irq // disable interrupts
+ disable_daif
str x0, [sp, #S_X0] // returned x0
ldr x1, [tsk, #TSK_TI_FLAGS] // re-check for syscall tracing
and x2, x1, #_TIF_SYSCALL_WORK
@@ -803,7 +803,7 @@ ret_fast_syscall:
enable_step_tsk x1, x2
kernel_exit 0
ret_fast_syscall_trace:
- enable_irq // enable interrupts
+ enable_daif
b __sys_trace_return_skipped // we already saved x0

/*
@@ -821,7 +821,7 @@ work_pending:
* "slow" syscall return path.
*/
ret_to_user:
- disable_irq // disable interrupts
+ disable_daif
ldr x1, [tsk, #TSK_TI_FLAGS]
and x2, x1, #_TIF_WORK_MASK
cbnz x2, work_pending
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 0bdc96c61bc0..8e6500c9471b 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -31,6 +31,7 @@
#include <linux/ratelimit.h>
#include <linux/syscalls.h>

+#include <asm/daifflags.h>
#include <asm/debug-monitors.h>
#include <asm/elf.h>
#include <asm/cacheflush.h>
@@ -756,9 +757,12 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
addr_limit_user_check();

if (thread_flags & _TIF_NEED_RESCHED) {
+ /* Unmask Debug and SError for the next task */
+ local_daif_restore(DAIF_PROCCTX_NOIRQ);
+
schedule();
} else {
- local_irq_enable();
+ local_daif_restore(DAIF_PROCCTX);

if (thread_flags & _TIF_UPROBE)
uprobe_notify_resume(regs);
@@ -775,7 +779,7 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
fpsimd_restore_current_state();
}

- local_irq_disable();
+ local_daif_mask();
thread_flags = READ_ONCE(current_thread_info()->flags);
} while (thread_flags & _TIF_WORK_MASK);
}
--
2.13.3