On Mon, Dec 7, 2015 at 1:51 PM, Andy Lutomirski <l...@kernel.org> wrote: > This is kind of like the 32-bit and compat code, except that I > preserved the fast path this time. I was unable to measure any > significant performance change on my laptop in the fast path. > > What do you all think?
For completeness, if I zap the fast path entirely (see attached), I lose 20 cycles (148 cycles vs 128 cycles) on Skylake. Switching between movq and pushq for stack setup makes no difference whatsoever, interestingly. I haven't tried to figure out exactly where those 20 cycles go. --Andy
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 1ab5362f241d..a97981f0d9ce 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -163,71 +163,17 @@ GLOBAL(entry_SYSCALL_64_after_swapgs) pushq %r9 /* pt_regs->r9 */ pushq %r10 /* pt_regs->r10 */ pushq %r11 /* pt_regs->r11 */ - sub $(6*8), %rsp /* pt_regs->bp, bx, r12-15 not saved */ + pushq %rbx /* pt_regs->rbx */ + pushq %rbp /* pt_regs->rbp */ + pushq %r12 /* pt_regs->r12 */ + pushq %r13 /* pt_regs->r13 */ + pushq %r14 /* pt_regs->r14 */ + pushq %r15 /* pt_regs->r15 */ - /* - * If we need to do entry work or if we guess we'll need to do - * exit work, go straight to the slow path. - */ - testl $_TIF_WORK_SYSCALL_ENTRY|_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS) - jnz entry_SYSCALL64_slow_path - -entry_SYSCALL_64_fastpath: - /* - * Easy case: enable interrupts and issue the syscall. If the syscall - * needs pt_regs, we'll call a stub that disables interrupts again - * and jumps to the slow path. - */ - TRACE_IRQS_ON - ENABLE_INTERRUPTS(CLBR_NONE) -#if __SYSCALL_MASK == ~0 - cmpq $__NR_syscall_max, %rax -#else - andl $__SYSCALL_MASK, %eax - cmpl $__NR_syscall_max, %eax -#endif - ja 1f /* return -ENOSYS (already in pt_regs->ax) */ - movq %r10, %rcx - call *sys_call_table_fastpath_64(, %rax, 8) - movq %rax, RAX(%rsp) -1: - - /* - * If we get here, then we know that pt_regs is clean for SYSRET64. - * If we see that no exit work is required (which we are required - * to check with IRQs off), then we can go straight to SYSRET64. - */ - DISABLE_INTERRUPTS(CLBR_NONE) - TRACE_IRQS_OFF - testl $_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS) - jnz 1f - - LOCKDEP_SYS_EXIT - TRACE_IRQS_ON /* user mode is traced as IRQs on */ - RESTORE_C_REGS - movq RSP(%rsp), %rsp - USERGS_SYSRET64 - -1: - /* - * The fast path looked good when we started, but something changed - * along the way and we need to switch to the slow path. Calling - * raise(3) will trigger this, for example. IRQs are off. - */ - TRACE_IRQS_ON - ENABLE_INTERRUPTS(CLBR_NONE) - SAVE_EXTRA_REGS - movq %rsp, %rdi - call syscall_return_slowpath /* returns with IRQs disabled */ - jmp return_from_SYSCALL_64 - -entry_SYSCALL64_slow_path: /* IRQs are off. */ - SAVE_EXTRA_REGS movq %rsp, %rdi call do_syscall_64 /* returns with IRQs disabled */ -return_from_SYSCALL_64: RESTORE_EXTRA_REGS TRACE_IRQS_IRETQ /* we're about to change IF */ @@ -305,14 +251,7 @@ opportunistic_sysret_failed: END(entry_SYSCALL_64) ENTRY(stub_ptregs_64) - /* - * Syscalls marked as needing ptregs that go through the fast path - * land here. We transfer to the slow path. - */ - DISABLE_INTERRUPTS(CLBR_NONE) - TRACE_IRQS_OFF - addq $8, %rsp - jmp entry_SYSCALL64_slow_path + ud2 END(stub_ptregs_64) /*
