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Dynamic binary translation
Sam King / Klara Nahrstedt
VMM environment
• Duplicate
– Virtual machine == physical machine
• Efficient
– Runs almost as fast s real machine
• Isolated
– VMM has control over resources
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Dynamic Binary Translation - Operating Systems Design | CS 423, Study notes of Operating Systems
Material Type: Notes; Class: Operating Systems Design; Subject: Computer Science; University: University of Illinois - Urbana-Champaign; Term: Unknown 1989;
Typology: Study notes
Pre 2010
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Dynamic binary translation
Sam King / Klara Nahrstedt
VMM environment
• Duplicate
– Virtual machine == physical machine
• Efficient
– Runs almost as fast s real machine
• Isolated
– VMM has control over resources
VMM is a simulator
• Simulator is software that simulates the
execution of a computer
while(1){ inst = mem[PC]; // fetch if(inst == add) { // decode // execute reg[inst.reg1] = reg[inst.reg2] + reg[inst.reg3]; PC++; } } // repeat
Simulator
- CPU state --- data structure within your sim.
- Includes registers, IDT, etc.
- Memory --- malloc
- Guest physical address 0 == beginning of malloc
- Disk --- file
- Disk block 1 = file offset 1*(size of disk block)
- Display --- window
- Within the simulator, software does not “know” it is
being simulated
Main issues
• Privileged instructions
• Instructions operate on physical state,
not virtual state
Privileged instructions
- CPU divided into supervisor and user modes
- Part of the CPU ISA only accessible by “supervisor”
code
- Allowing guest OS execute these would violate
isolation
- E.g., I/O instructions to write disk blocks
- Solution: run guest OS in user mode
- CPU user mode: only non-privileged inst
- CPU supervisor mode: all inst
Example: interrupt descriptor table (IDT)
• x86 processor have an IDT register
- CPU uses to find interrupt service routines
• Set the IDT register using the lidt instruction
• VMM must handle all interrupts to maintain
control
• Goal: allow the guest OS to set the virtual IDT
register without affecting the physical CPU
Guest OS priv. inst.
Hardware
VMM
Guest OS
Guest
app
Sensitive non-privileged instructions
• x86 has a number of instructions that
are sensitive, non-privileged instructions
– Causes physical state of CPU to leak
– Why is this a problem?
• What property does this violate?
Guest OS sensitive non-priv.
Hardware
VMM
Guest OS
Guest
app
Key insight
• Solution: simulate the OS, let user-mode
code run natively
- Simulation is flexible
- Can interpose on all instructions
- Problem: simulation is too slow
• Will work if guest OS calls SIDT, what about
guest user mode?
- Try it out for yourself! This is the basis for the “red
pill” VMM detection technique
Simulation flexibility
• Normal simulations flexible, slow
• Can we simulate fast, still flexible?
while(1){ inst = mem[PC]; // fetch if(inst == add) { // decode // execute reg[inst.reg1] = reg[inst.reg2] + reg[inst.reg3]; PC++; } } // repeat