From 3449b8c566e8ac173d882f0423cb66b579a2eb45 Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Thu, 25 Apr 2019 17:14:57 -0700
Subject: [PATCH] Implement the fast syscall challenge.

---
 inc/x86.h        | 19 +++++++++++++++++++
 kern/init.c      |  6 ++++++
 kern/trapentry.S | 15 +++++++++++++++
 lib/syscall.c    | 17 ++++++++++++++++-
 4 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/inc/x86.h b/inc/x86.h
index cc15ff4..bb6891d 100644
--- a/inc/x86.h
+++ b/inc/x86.h
@@ -3,6 +3,10 @@
 
 #include <inc/types.h>
 
+#define MSR_IA32_SYSENTER_CS 0x174
+#define MSR_IA32_SYSENTER_EIP 0x176
+#define MSR_IA32_SYSENTER_ESP 0x175
+
 static inline void
 breakpoint(void)
 {
@@ -261,4 +265,19 @@ xchg(volatile uint32_t *addr, uint32_t newval)
 	return result;
 }
 
+static inline void
+write_msr(uint32_t reg, uint32_t low, uint32_t high) {
+	asm volatile("wrmsr\n\t"
+			:: "c" (reg), "a" (low), "d" (high));
+}
+
+static inline void
+read_msr(uint32_t reg, uint32_t* low, uint32_t* high) {
+	uint32_t eax, edx;
+	asm volatile("rdmsr\n\t"
+			: "=a" (eax), "=d" (edx) : "c" (reg));
+	*low = eax;
+	*high = edx;
+}
+
 #endif /* !JOS_INC_X86_H */
diff --git a/kern/init.c b/kern/init.c
index b524643..0bdc695 100644
--- a/kern/init.c
+++ b/kern/init.c
@@ -3,6 +3,7 @@
 #include <inc/stdio.h>
 #include <inc/string.h>
 #include <inc/assert.h>
+#include <inc/x86.h>
 
 #include <kern/monitor.h>
 #include <kern/console.h>
@@ -11,6 +12,7 @@
 #include <kern/env.h>
 #include <kern/trap.h>
 
+void sysenter_handler();
 
 void
 i386_init(void)
@@ -34,6 +36,10 @@ i386_init(void)
 		"\33[34m" "r"
 		"\33[0m"  " Works!" "\n");
 
+	write_msr(MSR_IA32_SYSENTER_EIP, (uint32_t) sysenter_handler, 0);
+	write_msr(MSR_IA32_SYSENTER_ESP, KSTACKTOP, 0);
+	write_msr(MSR_IA32_SYSENTER_CS, GD_KT, 0);
+
 	// Lab 2 memory management initialization functions
 	mem_init();
 
diff --git a/kern/trapentry.S b/kern/trapentry.S
index 31b211f..f1d236e 100644
--- a/kern/trapentry.S
+++ b/kern/trapentry.S
@@ -43,6 +43,21 @@
 
 .text
 
+.globl sysenter_handler
+sysenter_handler:
+	push %ebp // holds env's stack pointer
+	push %esi // holds the env's return addr
+	push %edi
+	push %ebx
+	push %ecx
+	push %edx
+	push %eax
+	call syscall
+	add $0x14, %esp
+	pop %edx
+	pop %ecx
+	sysexit
+	
 /*
  * Lab 3: Your code here for generating entry points for the different traps.
  */
diff --git a/lib/syscall.c b/lib/syscall.c
index 8d28dda..08d7654 100644
--- a/lib/syscall.c
+++ b/lib/syscall.c
@@ -2,6 +2,7 @@
 
 #include <inc/syscall.h>
 #include <inc/lib.h>
+#include <inc/x86.h>
 
 static inline int32_t
 syscall(int num, int check, uint32_t a1, uint32_t a2, uint32_t a3, uint32_t a4, uint32_t a5)
@@ -37,10 +38,24 @@ syscall(int num, int check, uint32_t a1, uint32_t a2, uint32_t a3, uint32_t a4,
 	return ret;
 }
 
+int32_t
+fast_syscall(int num, uint32_t a1, uint32_t a2, uint32_t a3, uint32_t a4) {
+	asm volatile(
+			"push %%ebp\n\t"
+			"mov %%esp, %%ebp\n\t"
+			"lea syscall_ret_%=, %%esi\n\t"
+			"sysenter\n\t"
+			"syscall_ret_%=: pop %%ebp\n\t"
+			: "+a" (num)
+			: "d" (a1), "c" (a2), "b" (a3), "D" (a4)
+			: "esi");
+	return num;
+}
+
 void
 sys_cputs(const char *s, size_t len)
 {
-	syscall(SYS_cputs, 0, (uint32_t)s, len, 0, 0, 0);
+	fast_syscall(SYS_cputs, (uint32_t)s, len, 0, 0);
 }
 
 int