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Commit 29722cd4 authored by Denys Vlasenko's avatar Denys Vlasenko Committed by Ingo Molnar
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x86/asm/entry/64: Save R11 into pt_regs->flags on SYSCALL64 fastpath 



Before this patch, R11 was saved in pt_regs->r11.

Which looks natural, but requires messy shuffling to/from iret
frame whenever ptrace or e.g. sys_iopl() wants to modify flags -
because that's how this register is used by SYSCALL/SYSRET.

This patch saves R11 in pt_regs->flags, and uses that value for
the SYSRET64 instruction. Shuffling is eliminated.

FIXUP/RESTORE_TOP_OF_STACK are simplified.

stub_iopl is no longer needed: pt_regs->flags needs no fixing up.

Testing shows that syscall fast path is ~54.3 ns before
and after the patch (on 2.7 GHz Sandy Bridge CPU).

Signed-off-by: default avatarDenys Vlasenko <dvlasenk@redhat.com>
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/1425926364-9526-2-git-send-email-dvlasenk@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 3e1aa7cb

arch/x86/include/asm/calling.h

+14 −6
Original line number Diff line number Diff line
@@ -95,9 +95,11 @@ For 32-bit we have the following conventions - kernel is built with 
	CFI_ADJUST_CFA_OFFSET 15*8+\addskip

	.endm



	.macro SAVE_C_REGS_HELPER offset=0 rax=1 rcx=1 r8plus=1

	.if \r8plus

	.macro SAVE_C_REGS_HELPER offset=0 rax=1 rcx=1 r8910=1 r11=1

	.if \r11

	movq_cfi r11, 6*8+\offset

	.endif

	.if \r8910

	movq_cfi r10, 7*8+\offset

	movq_cfi r9,  8*8+\offset

	movq_cfi r8,  9*8+\offset
@@ -113,16 +115,19 @@ For 32-bit we have the following conventions - kernel is built with 
	movq_cfi rdi, 14*8+\offset

	.endm

	.macro SAVE_C_REGS offset=0

	SAVE_C_REGS_HELPER \offset, 1, 1, 1

	SAVE_C_REGS_HELPER \offset, 1, 1, 1, 1

	.endm

	.macro SAVE_C_REGS_EXCEPT_RAX_RCX offset=0

	SAVE_C_REGS_HELPER \offset, 0, 0, 1

	SAVE_C_REGS_HELPER \offset, 0, 0, 1, 1

	.endm

	.macro SAVE_C_REGS_EXCEPT_R891011

	SAVE_C_REGS_HELPER 0, 1, 1, 0

	SAVE_C_REGS_HELPER 0, 1, 1, 0, 0

	.endm

	.macro SAVE_C_REGS_EXCEPT_RCX_R891011

	SAVE_C_REGS_HELPER 0, 1, 0, 0

	SAVE_C_REGS_HELPER 0, 1, 0, 0, 0

	.endm

	.macro SAVE_C_REGS_EXCEPT_RAX_RCX_R11

	SAVE_C_REGS_HELPER 0, 0, 0, 1, 0

	.endm



	.macro SAVE_EXTRA_REGS offset=0
@@ -179,6 +184,9 @@ For 32-bit we have the following conventions - kernel is built with 
	.macro RESTORE_C_REGS_EXCEPT_R11

	RESTORE_C_REGS_HELPER 1,1,0,1,1

	.endm

	.macro RESTORE_C_REGS_EXCEPT_RCX_R11

	RESTORE_C_REGS_HELPER 1,0,0,1,1

	.endm

	.macro RESTORE_RSI_RDI

	RESTORE_C_REGS_HELPER 0,0,0,0,0

	.endm

arch/x86/kernel/entry_64.S

+11 −13
Original line number Diff line number Diff line
@@ -121,14 +121,12 @@ ENDPROC(native_usergs_sysret64) 
#endif



/*

 * C code is not supposed to know about undefined top of stack. Every time

 * a C function with an pt_regs argument is called from the SYSCALL based

 * fast path FIXUP_TOP_OF_STACK is needed.

 * C code is not supposed to know that the iret frame is not populated.

 * Every time a C function with an pt_regs argument is called from

 * the SYSCALL based fast path FIXUP_TOP_OF_STACK is needed.

 * RESTORE_TOP_OF_STACK syncs the syscall state after any possible ptregs

 * manipulation.

 */



	/* %rsp:at FRAMEEND */

	.macro FIXUP_TOP_OF_STACK tmp offset=0

	movq PER_CPU_VAR(old_rsp),\tmp

	movq \tmp,RSP+\offset(%rsp)
@@ -136,15 +134,13 @@ ENDPROC(native_usergs_sysret64) 
	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 R11+\offset(%rsp),\tmp  /* get eflags */

	movq \tmp,EFLAGS+\offset(%rsp)

	movq EFLAGS+\offset(%rsp),\tmp /* ditto for rflags->r11 */

	movq \tmp,R11+\offset(%rsp)

	.endm



	.macro RESTORE_TOP_OF_STACK tmp offset=0

	movq RSP+\offset(%rsp),\tmp

	movq \tmp,PER_CPU_VAR(old_rsp)

	movq EFLAGS+\offset(%rsp),\tmp

	movq \tmp,R11+\offset(%rsp)

	.endm



/*
@@ -257,9 +253,10 @@ GLOBAL(system_call_after_swapgs) 
	 */

	ENABLE_INTERRUPTS(CLBR_NONE)

	ALLOC_PT_GPREGS_ON_STACK 8		/* +8: space for orig_ax */

	SAVE_C_REGS_EXCEPT_RAX_RCX

	SAVE_C_REGS_EXCEPT_RAX_RCX_R11

	movq	$-ENOSYS,RAX(%rsp)

	movq_cfi rax,ORIG_RAX

	movq	%r11,EFLAGS(%rsp)

	movq	%rcx,RIP(%rsp)

	CFI_REL_OFFSET rip,RIP

	testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP)
@@ -277,7 +274,7 @@ system_call_fastpath: 
	movq %rax,RAX(%rsp)

/*

 * Syscall return path ending with SYSRET (fast path)

 * Has incomplete stack frame and undefined top of stack.

 * Has incompletely filled pt_regs, iret frame is also incomplete.

 */

ret_from_sys_call:

	testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP)
@@ -291,9 +288,10 @@ ret_from_sys_call: 
	 * sysretq will re-enable interrupts:

	 */

	TRACE_IRQS_ON

	RESTORE_C_REGS_EXCEPT_RCX

	RESTORE_C_REGS_EXCEPT_RCX_R11

	movq	RIP(%rsp),%rcx

	CFI_REGISTER	rip,rcx

	movq	EFLAGS(%rsp),%r11

	/*CFI_REGISTER	rflags,r11*/

	movq	PER_CPU_VAR(old_rsp), %rsp

	/*