[PATCH] arm64: syscall: unmask DAIF for tracing status

[Guo Hui]

The following code:
static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
                            const syscall_fn_t syscall_table[])
{
    ...

    if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
        local_daif_mask();
        flags = read_thread_flags(); -------------------------------- A
        if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
             return;
        local_daif_restore(DAIF_PROCCTX);
     }

trace_exit:
     syscall_trace_exit(regs); ------- B
}

1. The flags in the if conditional statement should be used
in the same way as the flags in the function syscall_trace_exit,
because DAIF is not shielded in the function syscall_trace_exit,
so when the flags are obtained in line A of the code,
there is no need to shield DAIF.
Don't care about the modification of flags after line A.

2. Masking DAIF caused syscall performance to deteriorate by about 10%.
The Unixbench single core syscall performance data is as follows:

Machine: Kunpeng 920

Mask DAIF:
System Call Overhead                    1172314.1 lps   (10.0 s, 7 samples)

System Benchmarks Partial Index          BASELINE       RESULT    INDEX
System Call Overhead                      15000.0    1172314.1    781.5
                                                               ========
System Benchmarks Index Score (Partial Only)                      781.5

Unmask DAIF:
System Call Overhead                    1287944.6 lps   (10.0 s, 7 samples)

System Benchmarks Partial Index          BASELINE       RESULT    INDEX
System Call Overhead                      15000.0    1287944.6    858.6
                                                               ========
System Benchmarks Index Score (Partial Only)                      858.6

Signed-off-by: Guo Hui <guohui at uniontech.com>
---
 arch/arm64/kernel/syscall.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index da84cf855c44..5a668d7f3c1f 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -147,11 +147,9 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	 * exit regardless, as the old entry assembly did.
 	 */
 	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
-		local_daif_mask();
 		flags = read_thread_flags();
 		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
 			return;
-		local_daif_restore(DAIF_PROCCTX);
 	}
 
 trace_exit:
-- 
2.20.1