Unmodified Device Driver Reuse and Improved System Dependability via Virtual Machines J. LeVasseur V. Uhlig J. Stoess S. G¨otz University of Karlsruhe, Germany Presented by: Aaron Beach Northwestern University

Outline 1. Intro 2. General Approach to Driver Re-use – - Basic Principles – - Architecture and Design Overview – - The Virtual Machine 3. Evaluation – - CPU and memory overhead – - Comparison with native Linux

Intro Basic idea: – - Run device drivers within their native OS in a virtual machine. – - Provide easy interface for client OS to use Why reuse driver code (or binary) – - Device driver code accounts for a large percentage of total operating system code – - Can enforce proper protection and modularity between device drivers and OS

Driver Design 1. Basic Principles 2. Architecture and Design Overview 3. The Virtual Machine 4. More Design Features 5. Biggest Design Issues - Memory - I/O - Other Problems (timing, Sharing Resources)

Driver Design Basic Principles for Reuse Resource Delegation – - Bulk Resources only, page level easier to standardize Separation of Name Spaces – - Within its own address space (proper protection) Separation of Privilege Secure Isolation – - Proper memory management, careful sharing Common API – - General necessity of reuse and standardization

Architecture Design Overview DD/OS – - Device Driver OS Virtual Machines – - Protection and Abstraction domain Client – - Uses Drivers via through the VM Translation Module – - The interface (glue) between client and the DD/OS

The Virtual Machine The Hyperviser – - Base kernel with ultimate privileges The VMM – - Virtualization layer, actually manages the Vms Device Management is direct (pass-through) – - i.e. VM-OS is given direct access to devices – OS -> Device OS <- VM <- Translation Module <- Client

Design Features: Client and Dependability Client interface provided by translation module – - Uses interrupts to be invoked and to respond Modularity – - Device failure and crashes are contained within VM – - VM can be rebooted VM Isolation – - VM enforces isolation, even if device does not

Issue: Memory DMA and VMM address translation – - DMA can't work directly in a VM because there is an extra layer of address translation and the DMA would try to directly access the wrong memory Solutions? – - DMA address translation is always done by VMM – - To give VM direct access to DMA, then the VM must be mapped directly to hardware addresses This creates a conflict of performance vs trust – - If there is trust then less protection is needed – - Otherwise, Translation must be used and the translations must stay constant during DMA... – - This also requires that pinning be governed

Issues: I/O, Resource Consumption, Timing I/O – - PCI bus is time multi-plexed... hurts performance Resource Consumption – - Overhead and Driver Resource Consumption extends beyond the VM – - Some efficiency can be gained by sharing memory footprints and VMM memory management Timing – - System may pre-empt real-time critical drivers – - Solution involves carrying VM interrupt disable actual machine interrupt disabling

Evaluation: Overhead Resources

CPU and Bandwidth Native Linux vs. L4 Paravirtualization

Conclusions Main Point: Driver Reuse Unmodified drivers Running in native OS Smaller implementation effort than rewriting Dependability (protection and modularity) Achieved with 3-8% overhead