arm64: Mask all exceptions during kernel_exit

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>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f5e851e..eff4a07 100644 (file)
--- 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 0bdc96c..8e6500c 100644 (file)
--- 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);
 }