On Tue, Jun 20, 2017 at 01:31:14PM -0700, Thomas Garnier wrote: > On Tue, Jun 20, 2017 at 1:18 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Wed, Jun 14, 2017 at 6:12 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote: > >> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S > >> index eb5cd77bf1d8..e33c32d56193 100644 > >> --- a/arch/arm/kernel/entry-common.S > >> +++ b/arch/arm/kernel/entry-common.S > >> @@ -41,7 +41,9 @@ ret_fast_syscall: > >> UNWIND(.cantunwind ) > >> disable_irq_notrace @ disable interrupts > >> ldr r1, [tsk, #TI_FLAGS] @ re-check for syscall tracing > >> - tst r1, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK > >> + tst r1, #_TIF_SYSCALL_WORK > >> + bne fast_work_pending > >> + tst r1, #_TIF_WORK_MASK > > > > (IIUC) MOV32 is 2 cycles (MOVW, MOVT), and each TST above is 1 cycle > > and each BNE is 1 cycle (when not taken). So: > > > > mov32 r2, #_TIF_SYSCALL_WORK | _TIF_WORK_MASK > > tst r1, r2 > > bne fast_work_pending > > > > is 4 cycles and tst, bne, tst, bne is also 4 cycles. Would mov32 be > > more readable (since it keeps the flags together)? > > I guess it would be more readable. Any opinion from the arm folks? The mov32 sequence is probably better, but statically attributing cycles on a per instruction basis is pretty futile on modern CPUs. Will -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
