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Date:	Sun, 5 Nov 2006 02:18:47 -0500
From:	Chuck Ebbert <76306.1226@...puserve.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	Andi Kleen <ak@...e.de>, Linus Torvalds <torvalds@...l.org>,
	Zachary Amsden <zach@...are.com>
Subject: [rfc patch] i386: only restore eflags if necessary on task
  switch

Save eflags during task switch but only restore them if the next
task has different values that affect system operation.

Original idea by Zach Amsden.

Signed-off-by: Chuck Ebbert <76306.1226@...puserve.com>

---

 arch/i386/kernel/ioport.c    |    8 ++++----
 arch/i386/kernel/process.c   |    6 +++---
 include/asm-i386/processor.h |   30 ++++++++++++++++++++----------
 include/asm-i386/system.h    |   12 +++++++-----
 4 files changed, 34 insertions(+), 22 deletions(-)

--- 2.6.19-rc4-32smp.orig/include/asm-i386/processor.h
+++ 2.6.19-rc4-32smp/include/asm-i386/processor.h
@@ -143,6 +143,12 @@ static inline void detect_ht(struct cpui
 #define X86_EFLAGS_VIP	0x00100000 /* Virtual Interrupt Pending */
 #define X86_EFLAGS_ID	0x00200000 /* CPUID detection flag */
 
+/*
+ * EFLAGS bits that affect system operation
+ */
+#define X86_EFLAGS_SYSTEM ~(X86_EFLAGS_OF | X86_EFLAGS_SF | X86_EFLAGS_ZF | \
+			    X86_EFLAGS_AF | X86_EFLAGS_PF | X86_EFLAGS_ZF)
+
 static inline void __cpuid(unsigned int *eax, unsigned int *ebx,
 			   unsigned int *ecx, unsigned int *edx)
 {
@@ -449,6 +455,7 @@ struct thread_struct {
 	unsigned long	sysenter_cs;
 	unsigned long	eip;
 	unsigned long	esp;
+ 	unsigned long	eflags;
 	unsigned long	fs;
 	unsigned long	gs;
 /* Hardware debugging registers */
@@ -464,7 +471,6 @@ struct thread_struct {
 	unsigned int		saved_fs, saved_gs;
 /* IO permissions */
 	unsigned long	*io_bitmap_ptr;
- 	unsigned long	iopl;
 /* max allowed port in the bitmap, in bytes: */
 	unsigned long	io_bitmap_max;
 };
@@ -521,20 +527,24 @@ static inline void load_esp0(struct tss_
 			: /* no output */			\
 			:"r" (value))
 
+static inline unsigned get_eflags(void) {
+	unsigned eflags;
+
+	asm volatile ("pushfl ; popl %0" : "=g" (eflags));
+
+	return eflags;
+}
+
+static inline void set_eflags(unsigned eflags) {
+	asm volatile ("pushl %0 ; popfl" : : "g" (eflags));
+}
+
 /*
  * Set IOPL bits in EFLAGS from given mask
  */
 static inline void set_iopl_mask(unsigned mask)
 {
-	unsigned int reg;
-	__asm__ __volatile__ ("pushfl;"
-			      "popl %0;"
-			      "andl %1, %0;"
-			      "orl %2, %0;"
-			      "pushl %0;"
-			      "popfl"
-				: "=&r" (reg)
-				: "i" (~X86_EFLAGS_IOPL), "r" (mask));
+	set_eflags((get_eflags() & ~X86_EFLAGS_IOPL) | (mask & X86_EFLAGS_IOPL));
 }
 
 /* Forward declaration, a strange C thing */
--- 2.6.19-rc4-32smp.orig/arch/i386/kernel/process.c
+++ 2.6.19-rc4-32smp/arch/i386/kernel/process.c
@@ -681,10 +681,10 @@ struct task_struct fastcall * __switch_t
 		loadsegment(gs, next->gs);
 
 	/*
-	 * Restore IOPL if needed.
+	 * Restore eflags if system flags are different in next task.
 	 */
-	if (unlikely(prev->iopl != next->iopl))
-		set_iopl_mask(next->iopl);
+	if (unlikely((get_eflags() ^ next->eflags) & X86_EFLAGS_SYSTEM))
+		set_eflags(next->eflags);
 
 	/*
 	 * Now maybe handle debug registers and/or IO bitmaps
--- 2.6.19-rc4-32smp.orig/include/asm-i386/system.h
+++ 2.6.19-rc4-32smp/include/asm-i386/system.h
@@ -14,23 +14,25 @@ extern struct task_struct * FASTCALL(__s
 /*
  * Saving eflags is important. It switches not only IOPL between tasks,
  * it also protects other tasks from NT leaking through sysenter etc.
+ * (eflags will be restored in __switch_to() only if necessary.)
  */
 #define switch_to(prev,next,last) do {					\
 	unsigned long esi,edi;						\
 	asm volatile("pushfl\n\t"		/* Save flags */	\
+		     "popl %2\n\t"					\
 		     "pushl %%ebp\n\t"					\
 		     "movl %%esp,%0\n\t"	/* save ESP */		\
-		     "movl %5,%%esp\n\t"	/* restore ESP */	\
+		     "movl %6,%%esp\n\t"	/* restore ESP */	\
 		     "movl $1f,%1\n\t"		/* save EIP */		\
-		     "pushl %6\n\t"		/* restore EIP */	\
+		     "pushl %7\n\t"		/* restore EIP */	\
 		     "jmp __switch_to\n"				\
 		     "1:\t"						\
-		     "popl %%ebp\n\t"					\
-		     "popfl"						\
+		     "popl %%ebp"					\
 		     :"=m" (prev->thread.esp),"=m" (prev->thread.eip),	\
+		      "=m" (prev->thread.eflags),			\
 		      "=a" (last),"=S" (esi),"=D" (edi)			\
 		     :"m" (next->thread.esp),"m" (next->thread.eip),	\
-		      "2" (prev), "d" (next));				\
+		      "3" (prev), "d" (next));				\
 } while (0)
 
 #define _set_base(addr,base) do { unsigned long __pr; \
--- 2.6.19-rc4-32smp.orig/arch/i386/kernel/ioport.c
+++ 2.6.19-rc4-32smp/arch/i386/kernel/ioport.c
@@ -137,7 +137,7 @@ asmlinkage long sys_iopl(unsigned long u
 	volatile struct pt_regs * regs = (struct pt_regs *) &unused;
 	unsigned int level = regs->ebx;
 	unsigned int old = (regs->eflags >> 12) & 3;
-	struct thread_struct *t = &current->thread;
+	unsigned int iopl;
 
 	if (level > 3)
 		return -EINVAL;
@@ -146,8 +146,8 @@ asmlinkage long sys_iopl(unsigned long u
 		if (!capable(CAP_SYS_RAWIO))
 			return -EPERM;
 	}
-	t->iopl = level << 12;
-	regs->eflags = (regs->eflags & ~X86_EFLAGS_IOPL) | t->iopl;
-	set_iopl_mask(t->iopl);
+	iopl = level << 12;
+	regs->eflags = (regs->eflags & ~X86_EFLAGS_IOPL) | iopl;
+	set_iopl_mask(iopl);
 	return 0;
 }
-- 
Chuck
-
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