Semester7
Notes of courses done/attended in semester 7 in college
Lecture 15
Video
link
User and Kernel Mode
Virtual MAchine Monitor
- is also an OS on top of which multiple os are running
Priviledged instructions
- if executed in user mode will result in trap
Sensitive Instructions
Control Sensitive instructions
- that change amount of resources, change processor mode etc etc
Behavior Sensitive instructions
- disable some interrupts, generally done by os
- loading physical address (user only sees virtual instructions and not physical)
Efficient VVMs : Popek and Gold Theorem 1
- what is relation b/w sensitive and priveledged instructions?
- if all sensitive instructions is subset of priveledged, then easy for vmmt o be built
- it has to not look into source code of program
- if any program runs ganda instruction trap hoga and it will get to know
- vmms does not have to worry, os kuch bhi execute kare, if kuch sensitive hua, toh usko pata chal jayega
- but if subset nahi hai, Sensitive - Priviledged = critical instructions
- prob kya hai?
- sensitive mtlb effecting resource alloation to processes
- agar priveledged nahi hua, trap nahi hoga agar user program ne run kia
- say SPT run karra OS1 on top of VMM (set cpu timer)
- OS1 runs in user mode and VMM in kernel mode
- yeh priveledged hai toh os isko bata dega and yeh os1 ke lie yeh kam kardega dekhke kisi tarah
- say popf instructions hai, os execute karta toh thik hai, bcz properly karega
- if user karta, koi trap nahi generate hota but replaced by a nop
- aba gar OS1 yeh karta execute, VMM ko pata hi nahi chala aisa kuch
- sensitive tha but priviledged nahi
IA-32/x86 Critical Instructions
- these put a load on VVM, it has to look at source code as well
- 18 sensitive, unpriveledged instructions
Efficient VVM
- due to presence of critical instructions, every thing has to go through VVM, directly h/w pe nahi execute nahi kar sakta
- 2 strategies
- Binary translation
- look every instruction, and put trap before critical instructions
- Paravirtualization
- guest os source code level
- modify it to insert interrups/traps there
System VMs
- bare metal h/w = type 1
- directly execurting on h/w
- hypervisor executing on h/w
- mulitple VM support efficiently
- guest os gets illuysion of directly running on h/w
- Hosted VM
- already installed hai ek OS
- Os provides a virtualization s/w on top of which I execute diff OSs, type 2
System VMs - Paravirtuyalization
- full virtualization => Guest os directly sees h/w (gets illusion)
- para me => it is aware of VMM is in between
- so executes system calls on it(hypercall in case of xen)
- modify source code of OS for para
- linux kernel ko modify kardeta so that critical isntructions ke lie scan na karna pade, it inserts hypercall procedures