sectionXen [69f17142]

Why is it important to provide timers?

• Because OS's aren't used to being preempted! We want to provide the OS real timers so they can do actual things
• Timing estimates, for example from TCP

Hypervisors are meant to be tiny, has direct machine access

• Hypervisor control node actually runs as a VM

Changes made to make virtualization easier

• Guest OS runs at 1, which works nicely for CPU x86 priv rings

• when the guest starts to run and gets scheduled, it communicates with the hypervisor to register a set of addresses

  • When there's a syscall, jump to this part in the OS

• GuestOSs have realtime and virtual time

  • real time for congestion
  • virtual time for when you get descheduled
• Hypercalls manages guests to the OS

Paging

• Virtualizing the MMU is very difficult, since the MMU is allowed to walk the page tables

• Xen allows the guest to maintain its own page tables, but Xen checks all modifications to the page tables

  • Software managed TLB or TLB with address space modifiers, but x86 doesn't have this
• everyone manages their own page tables
• minimized involvement for xen
• So Xen lives in the unused memory space

• When guest operating systems creates tables, they create the table ds with address space, any memory that's required is part of that address space

  • when they register the page table with xen, then it's read only
  • Xen does recieves updates, it can batch em
• Guest runs at 1