From 1e423bff959e48166f5b7efca01fdb0dbdf05846 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Thu, 28 Jan 2016 15:11:28 -0800 Subject: [PATCH] x86/entry/64: Migrate the 64-bit syscall slow path to C This is more complicated than the 32-bit and compat cases because it preserves an asm fast path for the case where the callee-saved regs aren't needed in pt_regs and no entry or exit work needs to be done. This appears to slow down fastpath syscalls by no more than one cycle on my Skylake laptop. Signed-off-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: Frederic Weisbecker Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/ce2335a4d42dc164b24132ee5e8c7716061f947b.1454022279.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/entry/common.c | 26 +++++++++ arch/x86/entry/entry_64.S | 117 +++++++++++++------------------------- 2 files changed, 65 insertions(+), 78 deletions(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 03663740c866..75175f92f462 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -344,6 +344,32 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs) prepare_exit_to_usermode(regs); } +#ifdef CONFIG_X86_64 +__visible void do_syscall_64(struct pt_regs *regs) +{ + struct thread_info *ti = pt_regs_to_thread_info(regs); + unsigned long nr = regs->orig_ax; + + local_irq_enable(); + + if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY) + nr = syscall_trace_enter(regs); + + /* + * NB: Native and x32 syscalls are dispatched from the same + * table. The only functional difference is the x32 bit in + * regs->orig_ax, which changes the behavior of some syscalls. + */ + if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) { + regs->ax = sys_call_table[nr & __SYSCALL_MASK]( + regs->di, regs->si, regs->dx, + regs->r10, regs->r8, regs->r9); + } + + syscall_return_slowpath(regs); +} +#endif + #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) /* * Does a 32-bit syscall. Called with IRQs on and does all entry and diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index cb5d940a7abd..567aa522ac0a 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -145,17 +145,11 @@ GLOBAL(entry_SYSCALL_64_after_swapgs) movq %rsp, PER_CPU_VAR(rsp_scratch) movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp + TRACE_IRQS_OFF + /* Construct struct pt_regs on stack */ pushq $__USER_DS /* pt_regs->ss */ pushq PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */ - /* - * Re-enable interrupts. - * We use 'rsp_scratch' as a scratch space, hence irq-off block above - * must execute atomically in the face of possible interrupt-driven - * task preemption. We must enable interrupts only after we're done - * with using rsp_scratch: - */ - ENABLE_INTERRUPTS(CLBR_NONE) pushq %r11 /* pt_regs->flags */ pushq $__USER_CS /* pt_regs->cs */ pushq %rcx /* pt_regs->ip */ @@ -171,9 +165,21 @@ GLOBAL(entry_SYSCALL_64_after_swapgs) pushq %r11 /* pt_regs->r11 */ sub $(6*8), %rsp /* pt_regs->bp, bx, r12-15 not saved */ - testl $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS) - jnz tracesys + /* + * 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 @@ -193,88 +199,43 @@ entry_SYSCALL_64_fastpath: movq %rax, RAX(%rsp) 1: -/* - * Syscall return path ending with SYSRET (fast path). - * Has incompletely filled pt_regs. - */ - LOCKDEP_SYS_EXIT - /* - * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON, - * it is too small to ever cause noticeable irq latency. - */ - DISABLE_INTERRUPTS(CLBR_NONE) /* - * We must check ti flags with interrupts (or at least preemption) - * off because we must *never* return to userspace without - * processing exit work that is enqueued if we're preempted here. - * In particular, returning to userspace with any of the one-shot - * flags (TIF_NOTIFY_RESUME, TIF_USER_RETURN_NOTIFY, etc) set is - * very bad. + * 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 int_ret_from_sys_call_irqs_off /* Go to the slow path */ + jnz 1f - RESTORE_C_REGS_EXCEPT_RCX_R11 - movq RIP(%rsp), %rcx - movq EFLAGS(%rsp), %r11 + LOCKDEP_SYS_EXIT + TRACE_IRQS_ON /* user mode is traced as IRQs on */ + RESTORE_C_REGS movq RSP(%rsp), %rsp - /* - * 64-bit SYSRET restores rip from rcx, - * rflags from r11 (but RF and VM bits are forced to 0), - * cs and ss are loaded from MSRs. - * Restoration of rflags re-enables interrupts. - * - * NB: On AMD CPUs with the X86_BUG_SYSRET_SS_ATTRS bug, the ss - * descriptor is not reinitialized. This means that we should - * avoid SYSRET with SS == NULL, which could happen if we schedule, - * exit the kernel, and re-enter using an interrupt vector. (All - * interrupt entries on x86_64 set SS to NULL.) We prevent that - * from happening by reloading SS in __switch_to. (Actually - * detecting the failure in 64-bit userspace is tricky but can be - * done.) - */ USERGS_SYSRET64 -GLOBAL(int_ret_from_sys_call_irqs_off) +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) - jmp int_ret_from_sys_call - - /* Do syscall entry tracing */ -tracesys: SAVE_EXTRA_REGS movq %rsp, %rdi - call syscall_trace_enter - - /* - * Reload registers from stack in case ptrace changed them. - * We don't reload %rax because syscall_trace_enter() returned - * the value it wants us to use in the table lookup. - */ - RESTORE_C_REGS_EXCEPT_RAX -#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 /* fixup for C */ - call *sys_call_table(, %rax, 8) - movq %rax, RAX(%rsp) - RESTORE_EXTRA_REGS -1: - /* Use IRET because user could have changed pt_regs->foo */ + call syscall_return_slowpath /* returns with IRQs disabled */ + jmp return_from_SYSCALL_64 -/* - * Syscall return path ending with IRET. - * Has correct iret frame. - */ -GLOBAL(int_ret_from_sys_call) +entry_SYSCALL64_slow_path: + /* IRQs are off. */ SAVE_EXTRA_REGS movq %rsp, %rdi - call syscall_return_slowpath /* returns with IRQs disabled */ + call do_syscall_64 /* returns with IRQs disabled */ + +return_from_SYSCALL_64: RESTORE_EXTRA_REGS TRACE_IRQS_IRETQ /* we're about to change IF */ @@ -364,7 +325,7 @@ ENTRY(stub_ptregs_64) /* Called from fast path -- pop return address and jump to slow path */ popq %rax - jmp tracesys /* called from fast path */ + jmp entry_SYSCALL64_slow_path /* called from fast path */ 1: /* Called from C */ -- 2.45.2