echo input

• Application handles echoing
• Interrupt handler handles echoing

Buffering

• threads: Producer, consumer
• interrupt handler?

Duplicate all threads or just the calling thread if fork() called?

• What if the child calls exec just after the fork?
• What if child does not call exec and current threads including the thread calling fork are using mutexes.

• Duplicate all threads (POSIX - no, just the calling thread, but...)
• Solaris: fork (all threads duplicated) and fork1 (Only calling thread
• POSIX provides calls to ensure other threads than calling thread do not own mutex locks (duplicated in child process)

Low Level

• Handles organization of files on disks/media: data structure to find/allocate disk blocks
• File meta data: permissions, owner, times

Higer Level

E.g., Linux VFS (virtual file system)

• Interface for accessing a file system
• Support multiple file systems. Each one implements the vfs interface.

Window supports multiple file systems: e.g., FAT16, FAT32, NTFS

Linux VFS supports multiple file ext2, ext3, ext4, and the Windows file systems.

Linux Interface in C

Each file system has its own FileObject (below) and must provide the functions that are inserted into its FileObject's array of file_ops.

typedef struct {
  unsigned short refcount;
  struct file_ops *file_op;
  /* function pointers */
} FileObject;

Access at higher level is through an array of function pointers. Higher level uses same interface for all file systems, just a different FileObject instance.

Choices

• malloc from heap
• Map a new segment

An "efficient" isolated duplicate of a real machine.

Virtual Machine	Real Machine
User
Privileged	User
	Privileged

The virtual machine runs on the (real) processor.

• virtual user mode: (real) processor is in user mode.
• virtual privileged mode: (real) processor is in user mode.

On the real machine

Executing	Real Processor Mode	Action
privileged	user	traps to real os
privileged	privileged	executes fully
sensitive	user	executes (reduced) effect (no trap)
sensitive	privileged	executes (full) effect

This pops a value off the stack and uses it to set the flags register.

This is a sensitive instruction.

If the processor is in user mode, the bit in the flags register to disable interrupts will not be changed by popf regardless of the value popped off the stack.

• Three address categories:
  • (guest) virtual
  • (guest) real
  • (host) real
• Page tables
  • guest: maps (guest) virtual pages to (guest) real
  • host (vmm): maps (guest) real to (host) real
• Shadow Page table
  • maps: (guest) virtual pages to (host) real
  • composition of guest page table and host page table
  • used by hardware when guest executes to avoid extra level of indirection in address translation
  • Requires any attempt by guest to access/change its page table must trap to the vmm host. (update shadow page table)
  • hardware translation uses the shadow page table entries.

Virtualization: Guest OS runs unmodified on host vmm.

Paravirtualization: Small modificatinos of Guest to make virtualization simpler and/or more efficient.

Modifications include to guests some hypervisor calls.

A hypervisor call is a mechanism that allows a guest os to make calls to the vmm (virtual machine monitor) running on the real machine.

Xen (see the text) provides virtual machines using paravirtualization.

Xen (like VMWare) uses a standard operating system for control functions including controlling actual devices. But it must be modified somewhat to handle the hypervisor calls from virtual machines and their guest OS's.

Unlike VMWare, guests VMs are NOT run as processes under control of the standard os.

In Xen guests are run in Xen "domains" that communicate with the standard os via hypervisor calls.

Three examples of paravirtualization in Xen:

1. To avoid flushing the TLB (translation lookaside buffer: page table entry cache) when invoking the vmm, Xen mapped into the upper 64Mb of each VM address space.
2. Guest allowed to allocated pages, just check that it didn't violate protection restrictions.
3. To protect guest OS from user programs, Xen takes advantage of Intel processor's 4 protection levels:
   • Most OS's running on Intel only use level 0 (kernel) and level 3 (user)
   • Xen runs as VMM at level 0
   • Guest OS runs at level 1
   • Applications run at level 3

The operating systems that can be used as the "standard os" supported by Xen currently includes recent versions of Linux and NetBSD.

To simplify I/O, a separate domain can be used just to handle a device (perhaps running at a lower privilege level, but not at user level; e.g. level 1) - sometimes called a "driver domain".

(In Xen, "domain" == virtual machine)

• Driver domains run physical device drivers
• Regular guest domains use simple virtual drivers and communicate with driver domains or the standard domain (how?).
• Data is exchanged between guest and driver domains by page remapping; that is, no copying of data.

Linux has optimizations for:

• network interface to directly store packets at different memory locations to avoid having to copy from buffers to the network protocol functions that process the packets. Not Xen. This results in up to 4 times as many L2 cache misses for Xen compared to native Linux.
• Data TLB misses. Linux also has optimizations here not currently addressed in Xen. Causes 12 - 24 times as many Data TLB misses

The functions you are to write to build your uthreads library naturally fall into 2 groups: the synchronization functions and the thread management functions (creation, scheduling, etc.)

Here is a suggested guide for first implementing and testing the management functions.

Once you have those working, you can implement and test the second group.