linux-kernel - [PATCH 1/3] x86/asm/entry/32: Massage SYSENTER32 fast path to be nearly identical to SYSCALL32

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Date:	Fri, 24 Jul 2015 15:47:46 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Krzysztof A. Sobiecki" <sobkas@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] x86/asm/entry/32: Massage SYSENTER32 fast path to be nearly identical to SYSCALL32

This change swaps a few instructions in final register restoring/zeroing
section of SYSENTER fast path, and adds/deletes a few empty lines.

After this, the difference between SYSENTER and SYCALL fast paths
(after the prologue which saved pt_regs) is very small:
they differ merely in the choice of register to hold arg6 (EBP or R9)
and in the value of EDX on exit: SYSENTER ABI doesn't need to preserve it,
so it is zeroed. SYSCALL preserves it:

       |(prologue is different)
       | 	orl	$TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
       | 	testl	$_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
       |-	jnz	sysenter_tracesys
       |+	jnz	cstar_tracesys
       |
       |-sysenter_do_call:
       |+cstar_do_call:
       | 	/* 32-bit syscall -> 64-bit C ABI argument conversion */
       | 	movl	%edi, %r8d		/* arg5 */
       |-	movl	%ebp, %r9d		/* arg6 */
       |+	/* r9 already loaded */		/* arg6 */
       | 	xchg	%ecx, %esi		/* rsi:arg2, rcx:arg4 */
       | 	movl	%ebx, %edi		/* arg1 */
       | 	movl	%edx, %edx		/* arg3 (zero extension) */
       |
       |-sysenter_dispatch:
       |+cstar_dispatch:
       | 	cmpq	$(IA32_NR_syscalls-1), %rax
       | 	ja	1f
       | 	call	*ia32_sys_call_table(, %rax, 8)
       |@@ -19,15 +19,15 @@
       | 	DISABLE_INTERRUPTS(CLBR_NONE)
       | 	TRACE_IRQS_OFF
       | 	testl	$_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
       |-	jnz	sysexit_audit
       |+	jnz	sysretl_audit
       |
       |-sysexit_from_sys_call:
       |+sysretl_from_sys_call:
       | 	andl	$~TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
       |+	movl	RDX(%rsp), %edx
       | 	movl	RSI(%rsp), %esi
       | 	movl	RDI(%rsp), %edi
       | 	movl	RIP(%rsp), %ecx
       | 	movl	EFLAGS(%rsp), %r11d
       |-	xorl	%edx, %edx
       | 	xorq	%r10, %r10
       | 	xorq	%r9, %r9
       | 	xorq	%r8, %r8
       |(the rest of fast path, up to final SYSRET32, is identical)

This is a preparatory change which allows to drop most of SYSENTER machinery
and make SYSENTER reuse SYSCALL code.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Ingo Molnar <mingo@...nel.org>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Krzysztof A. Sobiecki <sobkas@...il.com>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---
 arch/x86/entry/entry_64_compat.S | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 8997383..9f9dfa5 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -117,6 +117,7 @@ sysenter_do_call:
 	xchg	%ecx, %esi		/* rsi:arg2, rcx:arg4 */
 	movl	%ebx, %edi		/* arg1 */
 	movl	%edx, %edx		/* arg3 (zero extension) */
+
 sysenter_dispatch:
 	cmpq	$(IA32_NR_syscalls-1), %rax
 	ja	1f
@@ -127,6 +128,7 @@ sysenter_dispatch:
 	TRACE_IRQS_OFF
 	testl	$_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
 	jnz	sysexit_audit
+
 sysexit_from_sys_call:
 	/*
 	 * NB: SYSEXIT is not obviously safe for 64-bit kernels -- an
@@ -139,14 +141,14 @@ sysexit_from_sys_call:
 	 * with 'sysenter' and it uses the SYSENTER calling convention.
 	 */
 	andl	$~TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
-	movl	RIP(%rsp), %ecx		/* User %eip */
 	movl	RSI(%rsp), %esi
 	movl	RDI(%rsp), %edi
+	movl	RIP(%rsp), %ecx		/* User %eip */
+	movl	EFLAGS(%rsp), %r11d	/* User eflags */
 	xorl	%edx, %edx		/* Do not leak kernel information */
-	xorq	%r8, %r8
-	xorq	%r9, %r9
 	xorq	%r10, %r10
-	movl	EFLAGS(%rsp), %r11d	/* User eflags */
+	xorq	%r9, %r9
+	xorq	%r8, %r8
 	TRACE_IRQS_ON
 
 	/*
@@ -340,6 +342,7 @@ ENTRY(entry_SYSCALL_compat)
 1:	movl	(%r8), %r9d
 	_ASM_EXTABLE(1b, ia32_badarg)
 	ASM_CLAC
+
 	orl	$TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
 	testl	$_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
 	jnz	cstar_tracesys
@@ -355,7 +358,6 @@ cstar_do_call:
 cstar_dispatch:
 	cmpq	$(IA32_NR_syscalls-1), %rax
 	ja	1f
-
 	call	*ia32_sys_call_table(, %rax, 8)
 	movq	%rax, RAX(%rsp)
 1:
-- 
1.8.1.4

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