text: /* See COPYRIGHT for copyright information. */
  
  #include <inc/x86.h>
  #include <inc/mmu.h>
  #include <inc/error.h>
  #include <inc/string.h>
  #include <inc/assert.h>
  #include <inc/elf.h>
  
  #include <kern/env.h>
  #include <kern/pmap.h>
  #include <kern/trap.h>
  #include <kern/monitor.h>
  #include <kern/sched.h>
  #include <kern/cpu.h>
  #include <kern/spinlock.h>
  
  struct Env *envs = NULL;		// All environments
  static struct Env *env_free_list;	// Free environment list
  					// (linked by Env->env_link)
  
  #define ENVGENSHIFT	12		// >= LOGNENV
  
  // Global descriptor table.
  //
  // Set up global descriptor table (GDT) with separate segments for
  // kernel mode and user mode.  Segments serve many purposes on the x86.
  // We don't use any of their memory-mapping capabilities, but we need
  // them to switch privilege levels. 
  //
  // The kernel and user segments are identical except for the DPL.
  // To load the SS register, the CPL must equal the DPL.  Thus,
  // we must duplicate the segments for the user and the kernel.
  //
  // In particular, the last argument to the SEG macro used in the
  // definition of gdt specifies the Descriptor Privilege Level (DPL)
  // of that descriptor: 0 for kernel and 3 for user.
  //
  struct Segdesc gdt[NCPU + 5] =
  {
  	// 0x0 - unused (always faults -- for trapping NULL far pointers)
  	SEG_NULL,
  
  	// 0x8 - kernel code segment
  	[GD_KT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 0),
  
  	// 0x10 - kernel data segment
  	[GD_KD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 0),
  
  	// 0x18 - user code segment
  	[GD_UT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 3),
  
  	// 0x20 - user data segment
  	[GD_UD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 3),
  
  	// Per-CPU TSS descriptors (starting from GD_TSS0) are initialized
  	// in trap_init_percpu()
  	[GD_TSS0 >> 3] = SEG_NULL
  };
  
  struct Pseudodesc gdt_pd = {
  	sizeof(gdt) - 1, (unsigned long) gdt
  };
  
  //
  // Converts an envid to an env pointer.
  // If checkperm is set, the specified environment must be either the
  // current environment or an immediate child of the current environment.
  //
  // RETURNS
  //   0 on success, -E_BAD_ENV on error.
  //   On success, sets *env_store to the environment.
  //   On error, sets *env_store to NULL.
  //
  int
  envid2env(envid_t envid, struct Env **env_store, bool checkperm)
  {
  	struct Env *e;
  
  	// If envid is zero, return the current environment.
  	if (envid == 0) {
  		*env_store = curenv;
  		return 0;
  	}
  
  	// Look up the Env structure via the index part of the envid,
  	// then check the env_id field in that struct Env
  	// to ensure that the envid is not stale
  	// (i.e., does not refer to a _previous_ environment
  	// that used the same slot in the envs[] array).
  	e = &envs[ENVX(envid)];
  	if (e->env_status == ENV_FREE || e->env_id != envid) {
  		*env_store = 0;
  		return -E_BAD_ENV;
  	}
  
  	// Check that the calling environment has legitimate permission
  	// to manipulate the specified environment.
  	// If checkperm is set, the specified environment
  	// must be either the current environment
  	// or an immediate child of the current environment.
  	if (checkperm && e != curenv && e->env_parent_id != curenv->env_id) {
  		*env_store = 0;
  		return -E_BAD_ENV;
  	}
  
  	*env_store = e;
  	return 0;
  }
  
  // Mark all environments in 'envs' as free, set their env_ids to 0,
  // and insert them into the env_free_list.
  // Make sure the environments are in the free list in the same order
  // they are in the envs array (i.e., so that the first call to
  // env_alloc() returns envs[0]).
  //
  void
  env_init(void)
  {
  	// Set up envs array
  	// LAB 3: Your code here.
  	env_free_list = NULL;
  	for (int i = NENV - 1; i >= 0; --i) {
  		envs[i].env_id = 0;
  		envs[i].env_link = env_free_list;
  		env_free_list = &envs[i];
  		envs[i].env_status = ENV_FREE;
  	}
  
  	// Per-CPU part of the initialization
  	env_init_percpu();
  }
  
  // Load GDT and segment descriptors.
  void
  env_init_percpu(void)
  {
  	lgdt(&gdt_pd);
  	// The kernel never uses GS or FS, so we leave those set to
  	// the user data segment.
  	asm volatile("movw %%ax,%%gs" : : "a" (GD_UD|3));
  	asm volatile("movw %%ax,%%fs" : : "a" (GD_UD|3));
  	// The kernel does use ES, DS, and SS.  We'll change between
  	// the kernel and user data segments as needed.
  	asm volatile("movw %%ax,%%es" : : "a" (GD_KD));
  	asm volatile("movw %%ax,%%ds" : : "a" (GD_KD));
  	asm volatile("movw %%ax,%%ss" : : "a" (GD_KD));
  	// Load the kernel text segment into CS.
  	asm volatile("ljmp %0,$1f\n 1:\n" : : "i" (GD_KT));
  	// For good measure, clear the local descriptor table (LDT),
  	// since we don't use it.
  	lldt(0);
  }
  
  //
  // Initialize the kernel virtual memory layout for environment e.
  // Allocate a page directory, set e->env_pgdir accordingly,
  // and initialize the kernel portion of the new environment's address space.
  // Do NOT (yet) map anything into the user portion
  // of the environment's virtual address space.
  //
  // Returns 0 on success, < 0 on error.  Errors include:
  //	-E_NO_MEM if page directory or table could not be allocated.
  //
  static int
  env_setup_vm(struct Env *e)
  {
  	int i;
  	struct PageInfo *p = NULL;
  
  	// Allocate a page for the page directory
  	if (!(p = page_alloc(ALLOC_ZERO)))
  		return -E_NO_MEM;
  
  	// Now, set e->env_pgdir and initialize the page directory.
  	//
  	// Hint:
  	//    - The VA space of all envs is identical above UTOP
  	//	(except at UVPT, which we've set below).
  	//	See inc/memlayout.h for permissions and layout.
  	//	Can you use kern_pgdir as a template?  Hint: Yes.
  	//	(Make sure you got the permissions right in Lab 2.)
  	//    - The initial VA below UTOP is empty.
  	//    - You do not need to make any more calls to page_alloc.
  	//    - Note: In general, pp_ref is not maintained for
  	//	physical pages mapped only above UTOP, but env_pgdir
  	//	is an exception -- you need to increment env_pgdir's
  	//	pp_ref for env_free to work correctly.
  	//    - The functions in kern/pmap.h are handy.
  
  	// LAB 3: Your code here.
  	e->env_pgdir = page2kva(p);
  	memcpy(e->env_pgdir, kern_pgdir, PGSIZE);
  	++p->pp_ref;
  
  	// UVPT maps the env's own page table read-only.
  	// Permissions: kernel R, user R
  	e->env_pgdir[PDX(UVPT)] = PADDR(e->env_pgdir) | PTE_P | PTE_U;
  
  	return 0;
  }
  
  //
  // Allocates and initializes a new environment.
  // On success, the new environment is stored in *newenv_store.
  //
  // Returns 0 on success, < 0 on failure.  Errors include:
  //	-E_NO_FREE_ENV if all NENV environments are allocated
  //	-E_NO_MEM on memory exhaustion
  //
  int
  env_alloc(struct Env **newenv_store, envid_t parent_id)
  {
  	int32_t generation;
  	int r;
  	struct Env *e;
  
  	if (!(e = env_free_list))
  		return -E_NO_FREE_ENV;
  
  	// Allocate and set up the page directory for this environment.
  	if ((r = env_setup_vm(e)) < 0)
  		return r;
  
  	// Generate an env_id for this environment.
  	generation = (e->env_id + (1 << ENVGENSHIFT)) & ~(NENV - 1);
  	if (generation <= 0)	// Don't create a negative env_id.
  		generation = 1 << ENVGENSHIFT;
  	e->env_id = generation | (e - envs);
  
  	// Set the basic status variables.
  	e->env_parent_id = parent_id;
  	e->env_type = ENV_TYPE_USER;
  	e->env_status = ENV_RUNNABLE;
  	e->env_runs = 0;
  
  	// Clear out all the saved register state,
  	// to prevent the register values
  	// of a prior environment inhabiting this Env structure
  	// from "leaking" into our new environment.
  	memset(&e->env_tf, 0, sizeof(e->env_tf));
  
  	// Set up appropriate initial values for the segment registers.
  	// GD_UD is the user data segment selector in the GDT, and
  	// GD_UT is the user text segment selector (see inc/memlayout.h).
  	// The low 2 bits of each segment register contains the
  	// Requestor Privilege Level (RPL); 3 means user mode.  When
  	// we switch privilege levels, the hardware does various
  	// checks involving the RPL and the Descriptor Privilege Level
  	// (DPL) stored in the descriptors themselves.
  	e->env_tf.tf_ds = GD_UD | 3;
  	e->env_tf.tf_es = GD_UD | 3;
  	e->env_tf.tf_ss = GD_UD | 3;
  	e->env_tf.tf_esp = USTACKTOP;
  	e->env_tf.tf_cs = GD_UT | 3;
  	// You will set e->env_tf.tf_eip later.
  
  	// Enable interrupts while in user mode.
  	// LAB 4: Your code here.
  	e->env_tf.tf_eflags |= FL_IF;
  
  	// Clear the page fault handler until user installs one.
  	e->env_pgfault_upcall = 0;
  
  	// Also clear the IPC receiving flag.
  	e->env_ipc_recving = 0;
  
  	// commit the allocation
  	env_free_list = e->env_link;
  	*newenv_store = e;
  
  	// cprintf("[%08x] new env %08x\n", curenv ? curenv->env_id : 0, e->env_id);
  	return 0;
  }
  
  //
  // Allocate len bytes of physical memory for environment env,
  // and map it at virtual address va in the environment's address space.
  // Does not zero or otherwise initialize the mapped pages in any way.
  // Pages should be writable by user and kernel.
  // Panic if any allocation attempt fails.
  //
  static void
  region_alloc(struct Env *e, void *va, size_t len)
  {
  	// LAB 3: Your code here.
  	// (But only if you need it for load_icode.)
  	//
  	// Hint: It is easier to use region_alloc if the caller can pass
  	//   'va' and 'len' values that are not page-aligned.
  	//   You should round va down, and round (va + len) up.
  	//   (Watch out for corner-cases!)
  	uintptr_t va_begin = ROUNDDOWN((uintptr_t)va, PGSIZE);
  	uintptr_t va_end = ROUNDUP((uintptr_t)va + len, PGSIZE);
  	assert(va_begin <= va_end);
  
  	for (uintptr_t v = va_begin; v < va_end; v += PGSIZE) {
  		struct PageInfo *p = page_alloc(0);
  		assert(p);
  		assert(page_insert(e->env_pgdir, p, (void *)v, PTE_U | PTE_W) == 0);
  	}
  }
  
  //
  // Set up the initial program binary, stack, and processor flags
  // for a user process.
  // This function is ONLY called during kernel initialization,
  // before running the first user-mode environment.
  //
  // This function loads all loadable segments from the ELF binary image
  // into the environment's user memory, starting at the appropriate
  // virtual addresses indicated in the ELF program header.
  // At the same time it clears to zero any portions of these segments
  // that are marked in the program header as being mapped
  // but not actually present in the ELF file - i.e., the program's bss section.
  //
  // All this is very similar to what our boot loader does, except the boot
  // loader also needs to read the code from disk.  Take a look at
  // boot/main.c to get ideas.
  //
  // Finally, this function maps one page for the program's initial stack.
  //
  // load_icode panics if it encounters problems.
  //  - How might load_icode fail?  What might be wrong with the given input?
  //
  static void
  load_icode(struct Env *e, uint8_t *binary)
  {
  	// Hints:
  	//  Load each program segment into virtual memory
  	//  at the address specified in the ELF segment header.
  	//  You should only load segments with ph->p_type == ELF_PROG_LOAD.
  	//  Each segment's virtual address can be found in ph->p_va
  	//  and its size in memory can be found in ph->p_memsz.
  	//  The ph->p_filesz bytes from the ELF binary, starting at
  	//  'binary + ph->p_offset', should be copied to virtual address
  	//  ph->p_va.  Any remaining memory bytes should be cleared to zero.
  	//  (The ELF header should have ph->p_filesz <= ph->p_memsz.)
  	//  Use functions from the previous lab to allocate and map pages.
  	//
  	//  All page protection bits should be user read/write for now.
  	//  ELF segments are not necessarily page-aligned, but you can
  	//  assume for this function that no two segments will touch
  	//  the same virtual page.
  	//
  	//  You may find a function like region_alloc useful.
  	//
  	//  Loading the segments is much simpler if you can move data
  	//  directly into the virtual addresses stored in the ELF binary.
  	//  So which page directory should be in force during
  	//  this function?
  	//
  	//  You must also do something with the program's entry point,
  	//  to make sure that the environment starts executing there.
  	//  What?  (See env_run() and env_pop_tf() below.)
  
  	// LAB 3: Your code here.
  	struct Elf *ELFHDR = (struct Elf *)binary;
  	assert(ELFHDR->e_magic == ELF_MAGIC);
  
  	struct Proghdr *ph = (struct Proghdr *) (binary + ELFHDR->e_phoff);
  	struct Proghdr *eph = ph + ELFHDR->e_phnum;
  
  	lcr3(PADDR(e->env_pgdir));
  
  	for (; ph < eph; ++ph) {
  		if (ph->p_type == ELF_PROG_LOAD) {
  			region_alloc(e, (void *)ph->p_va, ph->p_memsz);
  			memset((void *)ph->p_va, 0, ph->p_memsz);
  			memcpy((void *)ph->p_va, binary + ph->p_offset, ph->p_filesz);
  		}
  	}
  
  	lcr3(PADDR(kern_pgdir));
  
  	e->env_tf.tf_eip = ELFHDR->e_entry;
  
  	// Now map one page for the program's initial stack
  	// at virtual address USTACKTOP - PGSIZE.
  
  	// LAB 3: Your code here.
  	region_alloc(e, (void *)(USTACKTOP - PGSIZE), PGSIZE);
  }
  
  //
  // Allocates a new env with env_alloc, loads the named elf
  // binary into it with load_icode, and sets its env_type.
  // This function is ONLY called during kernel initialization,
  // before running the first user-mode environment.
  // The new env's parent ID is set to 0.
  //
  void
  env_create(uint8_t *binary, enum EnvType type)
  {
  	// LAB 3: Your code here.
  	struct Env *e;
  	assert(env_alloc(&e, 0) == 0);
  	load_icode(e, binary);
  	e->env_type = type;
  
  	// If this is the file server (type == ENV_TYPE_FS) give it I/O privileges.
  	// LAB 5: Your code here.
  	if (e->env_type == ENV_TYPE_FS)
  		e->env_tf.tf_eflags |= FL_IOPL_MASK;
  }
  
  //
  // Frees env e and all memory it uses.
  //
  void
  env_free(struct Env *e)
  {
  	pte_t *pt;
  	uint32_t pdeno, pteno;
  	physaddr_t pa;
  
  	// If freeing the current environment, switch to kern_pgdir
  	// before freeing the page directory, just in case the page
  	// gets reused.
  	if (e == curenv)
  		lcr3(PADDR(kern_pgdir));
  
  	// Note the environment's demise.
  	// cprintf("[%08x] free env %08x\n", curenv ? curenv->env_id : 0, e->env_id);
  
  	// Flush all mapped pages in the user portion of the address space
  	static_assert(UTOP % PTSIZE == 0);
  	for (pdeno = 0; pdeno < PDX(UTOP); pdeno++) {
  
  		// only look at mapped page tables
  		if (!(e->env_pgdir[pdeno] & PTE_P))
  			continue;
  
  		// find the pa and va of the page table
  		pa = PTE_ADDR(e->env_pgdir[pdeno]);
  		pt = (pte_t*) KADDR(pa);
  
  		// unmap all PTEs in this page table
  		for (pteno = 0; pteno <= PTX(~0); pteno++) {
  			if (pt[pteno] & PTE_P)
  				page_remove(e->env_pgdir, PGADDR(pdeno, pteno, 0));
  		}
  
  		// free the page table itself
  		e->env_pgdir[pdeno] = 0;
  		page_decref(pa2page(pa));
  	}
  
  	// free the page directory
  	pa = PADDR(e->env_pgdir);
  	e->env_pgdir = 0;
  	page_decref(pa2page(pa));
  
  	// return the environment to the free list
  	e->env_status = ENV_FREE;
  	e->env_link = env_free_list;
  	env_free_list = e;
  }
  
  //
  // Frees environment e.
  // If e was the current env, then runs a new environment (and does not return
  // to the caller).
  //
  void
  env_destroy(struct Env *e)
  {
  	// If e is currently running on other CPUs, we change its state to
  	// ENV_DYING. A zombie environment will be freed the next time
  	// it traps to the kernel.
  	if (e->env_status == ENV_RUNNING && curenv != e) {
  		e->env_status = ENV_DYING;
  		return;
  	}
  
  	env_free(e);
  
  	if (curenv == e) {
  		curenv = NULL;
  		sched_yield();
  	}
  }
  
  
  //
  // Restores the register values in the Trapframe with the 'iret' instruction.
  // This exits the kernel and starts executing some environment's code.
  //
  // This function does not return.
  //
  void
  env_pop_tf(struct Trapframe *tf)
  {
  	// Record the CPU we are running on for user-space debugging
  	curenv->env_cpunum = cpunum();
  
  	asm volatile(
  		"\tmovl %0,%%esp\n"
  		"\tpopal\n"
  		"\tpopl %%es\n"
  		"\tpopl %%ds\n"
  		"\taddl $0x8,%%esp\n" /* skip tf_trapno and tf_errcode */
  		"\tiret\n"
  		: : "g" (tf) : "memory");
  	panic("iret failed");  /* mostly to placate the compiler */
  }
  
  //
  // Context switch from curenv to env e.
  // Note: if this is the first call to env_run, curenv is NULL.
  //
  // This function does not return.
  //
  void
  env_run(struct Env *e)
  {
  	// Step 1: If this is a context switch (a new environment is running):
  	//	   1. Set the current environment (if any) back to
  	//	      ENV_RUNNABLE if it is ENV_RUNNING (think about
  	//	      what other states it can be in),
  	//	   2. Set 'curenv' to the new environment,
  	//	   3. Set its status to ENV_RUNNING,
  	//	   4. Update its 'env_runs' counter,
  	//	   5. Use lcr3() to switch to its address space.
  	// Step 2: Use env_pop_tf() to restore the environment's
  	//	   registers and drop into user mode in the
  	//	   environment.
  
  	// Hint: This function loads the new environment's state from
  	//	e->env_tf.  Go back through the code you wrote above
  	//	and make sure you have set the relevant parts of
  	//	e->env_tf to sensible values.
  
  	// LAB 3: Your code here.
  	if (curenv) {
  		if (curenv->env_status == ENV_RUNNING) {
  			curenv->env_status = ENV_RUNNABLE;
  		}
  	}
  
  	curenv = e;
  	curenv->env_status = ENV_RUNNING;
  	curenv->env_runs++;
  	lcr3(PADDR(curenv->env_pgdir));
  	unlock_kernel();
  	env_pop_tf(&curenv->env_tf);
  
  	panic("env_run not yet implemented");
  }
  
  