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x86/asm/entry/64: Use PUSH instructions to build pt_regs on stack 

With this change, on SYSCALL64 code path we are now populating
pt_regs->cs, pt_regs->ss and pt_regs->rcx unconditionally and
therefore don't need to do that in FIXUP_TOP_OF_STACK.

We lose a number of large instructions there:

    text    data     bss     dec     hex filename
   13298       0       0   13298    33f2 entry_64_before.o
   12978       0       0   12978    32b2 entry_64.o

What's more important, we convert two "MOVQ $imm,off(%rsp)" to
"PUSH $imm" (the ones which fill pt_regs->cs,ss).

Before this patch, placing them on fast path was slowing it down
by two cycles: this form of MOV is very large, 12 bytes, and
this probably reduces decode bandwidth to one instruction per cycle
when CPU sees them.

Therefore they were living in FIXUP_TOP_OF_STACK instead (away
from fast path).

"PUSH $imm" is a small 2-byte instruction. Moving it to fast path does
not slow it down in my measurements.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1426785469-15125-3-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Denys Vlasenko 2015-03-19 18:17:47 +01:00 committed by Ingo Molnar
parent ef593260f0
commit 9ed8e7d860
1 changed files with 32 additions and 22 deletions
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54
arch/x86/kernel/entry_64.S
View File

@ -126,11 +126,8 @@ ENDPROC(native_usergs_sysret64)
* manipulation.
*/
.macro FIXUP_TOP_OF_STACK tmp offset=0
movq $__USER_DS,SS+\offset(%rsp)
movq $__USER_CS,CS+\offset(%rsp)
movq RIP+\offset(%rsp),\tmp /* get rip */
movq \tmp,RCX+\offset(%rsp) /* copy it to rcx as sysret would do */
movq EFLAGS+\offset(%rsp),\tmp /* ditto for rflags->r11 */
/* copy flags to r11 as sysret would do */
movq EFLAGS+\offset(%rsp),\tmp
movq \tmp,R11+\offset(%rsp)
.endm
@ -214,7 +211,6 @@ ENDPROC(native_usergs_sysret64)
* r9 arg5
* (note: r12-r15,rbp,rbx are callee-preserved in C ABI)
*
* Interrupts are off on entry.
* Only called from user space.
*
* When user can change pt_regs->foo always force IRET. That is because
@ -228,6 +224,12 @@ ENTRY(system_call)
CFI_DEF_CFA rsp,0
CFI_REGISTER rip,rcx
/*CFI_REGISTER rflags,r11*/
/*
* Interrupts are off on entry.
* We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
* it is too small to ever cause noticeable irq latency.
*/
SWAPGS_UNSAFE_STACK
/*
* A hypervisor implementation might want to use a label
@ -236,27 +238,35 @@ ENTRY(system_call)
*/
GLOBAL(system_call_after_swapgs)
/*
* We use 'rsp_scratch' as a scratch register, hence this block must execute
* atomically in the face of possible interrupt-driven task preemption,
* so we can enable interrupts only after we're done with using rsp_scratch:
*/
movq %rsp,PER_CPU_VAR(rsp_scratch)
movq PER_CPU_VAR(kernel_stack),%rsp
ALLOC_PT_GPREGS_ON_STACK 6*8 /* 6*8: space for orig_ax and iret frame */
movq %rcx,RIP(%rsp)
movq PER_CPU_VAR(rsp_scratch),%rcx
movq %r11,EFLAGS(%rsp)
movq %rcx,RSP(%rsp)
/* Construct struct pt_regs on stack */
pushq_cfi $__USER_DS /* pt_regs->ss */
pushq_cfi PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */
/*
* No need to follow this irqs off/on section - it's straight
* and short:
* 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)
movq_cfi rax,ORIG_RAX
SAVE_C_REGS_EXCEPT_RAX_RCX_R11
movq $-ENOSYS,RAX(%rsp)
CFI_REL_OFFSET rip,RIP
pushq_cfi %r11 /* pt_regs->flags */
pushq_cfi $__USER_CS /* pt_regs->cs */
pushq_cfi %rcx /* pt_regs->ip */
CFI_REL_OFFSET rip,0
pushq_cfi_reg rax /* pt_regs->orig_ax */
pushq_cfi_reg rdi /* pt_regs->di */
pushq_cfi_reg rsi /* pt_regs->si */
pushq_cfi_reg rdx /* pt_regs->dx */
pushq_cfi_reg rcx /* pt_regs->cx */
pushq_cfi $-ENOSYS /* pt_regs->ax */
pushq_cfi_reg r8 /* pt_regs->r8 */
pushq_cfi_reg r9 /* pt_regs->r9 */
pushq_cfi_reg r10 /* pt_regs->r10 */
sub $(7*8),%rsp /* pt_regs->r11,bp,bx,r12-15 not saved */
testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,SIZEOF_PTREGS)
jnz tracesys
system_call_fastpath: