linux-kernel - [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call

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Date:   Tue,  9 Jan 2018 17:03:21 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     tglx@...utronix.de
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, dwmw@...zon.co.uk, pjt@...gle.com,
        luto@...nel.org, peterz@...radead.org, thomas.lendacky@....com,
        tim.c.chen@...ux.intel.com, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, jikos@...nel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call

From: Andi Kleen <ak@...ux.intel.com>

Remove the partial stack frame in the 64bit syscall fast path.
In the next patch we want to clear the extra registers, which requires
to always save all registers. So remove the partial stack frame
in the syscall fast path and always save everything.

This actually simplifies the code because the ptregs stubs
are not needed anymore.

arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------------------
arch/x86/entry/syscall_64.c |  2 +-

Signed-off-by: Andi Kleen <ak@...ux.intel.com>
---
 arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------
 arch/x86/entry/syscall_64.c |  2 +-
 2 files changed, 5 insertions(+), 54 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 58dbf7a12a05..bbdfbdd817d6 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -234,7 +234,9 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
 	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 */
+	sub	$(6*8), %rsp
+	SAVE_EXTRA_REGS
+
 	UNWIND_HINT_REGS extra=0
 
 	/*
@@ -262,11 +264,6 @@ entry_SYSCALL_64_fastpath:
 	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
 	movq	%r10, %rcx
 
-	/*
-	 * This call instruction is handled specially in stub_ptregs_64.
-	 * It might end up jumping to the slow path.  If it jumps, RAX
-	 * and all argument registers are clobbered.
-	 */
 #ifdef CONFIG_RETPOLINE
 	movq	sys_call_table(, %rax, 8), %rax
 	call	__x86_indirect_thunk_rax
@@ -293,9 +290,7 @@ entry_SYSCALL_64_fastpath:
 	TRACE_IRQS_ON		/* user mode is traced as IRQs on */
 	movq	RIP(%rsp), %rcx
 	movq	EFLAGS(%rsp), %r11
-	addq	$6*8, %rsp	/* skip extra regs -- they were preserved */
-	UNWIND_HINT_EMPTY
-	jmp	.Lpop_c_regs_except_rcx_r11_and_sysret
+	jmp	syscall_return_via_sysret
 
 1:
 	/*
@@ -305,14 +300,12 @@ entry_SYSCALL_64_fastpath:
 	 */
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_ANY)
-	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 */
 
@@ -389,7 +382,6 @@ syscall_return_via_sysret:
 	/* rcx and r11 are already restored (see code above) */
 	UNWIND_HINT_EMPTY
 	POP_EXTRA_REGS
-.Lpop_c_regs_except_rcx_r11_and_sysret:
 	popq	%rsi	/* skip r11 */
 	popq	%r10
 	popq	%r9
@@ -420,47 +412,6 @@ syscall_return_via_sysret:
 	USERGS_SYSRET64
 END(entry_SYSCALL_64)
 
-ENTRY(stub_ptregs_64)
-	/*
-	 * Syscalls marked as needing ptregs land here.
-	 * If we are on the fast path, we need to save the extra regs,
-	 * which we achieve by trying again on the slow path.  If we are on
-	 * the slow path, the extra regs are already saved.
-	 *
-	 * RAX stores a pointer to the C function implementing the syscall.
-	 * IRQs are on.
-	 */
-	cmpq	$.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
-	jne	1f
-
-	/*
-	 * Called from fast path -- disable IRQs again, pop return address
-	 * and jump to slow path
-	 */
-	DISABLE_INTERRUPTS(CLBR_ANY)
-	TRACE_IRQS_OFF
-	popq	%rax
-	UNWIND_HINT_REGS extra=0
-	jmp	entry_SYSCALL64_slow_path
-
-1:
-	JMP_NOSPEC %rax				/* Called from C */
-END(stub_ptregs_64)
-
-.macro ptregs_stub func
-ENTRY(ptregs_\func)
-	UNWIND_HINT_FUNC
-	leaq	\func(%rip), %rax
-	jmp	stub_ptregs_64
-END(ptregs_\func)
-.endm
-
-/* Instantiate ptregs_stub for each ptregs-using syscall */
-#define __SYSCALL_64_QUAL_(sym)
-#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_stub sym
-#define __SYSCALL_64(nr, sym, qual) __SYSCALL_64_QUAL_##qual(sym)
-#include <asm/syscalls_64.h>
-
 /*
  * %rdi: prev task
  * %rsi: next task
diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c
index 9c09775e589d..ad1ae014f943 100644
--- a/arch/x86/entry/syscall_64.c
+++ b/arch/x86/entry/syscall_64.c
@@ -8,7 +8,7 @@
 #include <asm/syscall.h>
 
 #define __SYSCALL_64_QUAL_(sym) sym
-#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_##sym
+#define __SYSCALL_64_QUAL_ptregs(sym) sym
 
 #define __SYSCALL_64(nr, sym, qual) extern asmlinkage long __SYSCALL_64_QUAL_##qual(sym)(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
 #include <asm/syscalls_64.h>
-- 
2.14.3