Re: [PATCH 0/7] KVM: Kernel-based Virtual Machine

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Muli Ben-Yehuda wrote:
Hi,

Looks pretty interesting! some comments:

- patch 4/7 hasn't made it to the list?

Probably too big. It's also the ugliest. I'll split it and resend (not through thunderbird though... ate all my tabs!).


- it would be useful for reviewing this if you could post example code
  making use of the /dev/kvm interfaces - they seem fairly complex.

Working code is fairly hairy, since it's emulating a PC. That'll be on sourceforge once they approve my new project.

In general one does

 open("/dev/kvm")
 ioctl(KVM_SET_MEMORY_REGION) for main memory
 ioctl(KVM_SET_MEMORY_REGION) for the framebuffer
 ioctl(KVM_CREATE_VCPU) for the obvious reason
 if (debugger)
   ioctl(KVM_DEBUG_GUEST) to singlestep or breakpoint the guest
 while (1) {
    ioctl(KVM_RUN)
    switch (exit reason) {
        handle mmio, I/O etc. might call
           ioctl(KVM_INTERRUPT) to queue an external interrupt
           ioctl(KVM_{GET,SET}_{REGS,SREGS}) to query/modify registers
ioctl(KVM_GET_DIRTY_LOG) to see which guest memory pages have changed
    }

I have some simple test code, I'll clean it up and post it.

- why do it this way rather than through a virtual machine monitor
  such as Xen? what do you gain from having the virtual machines
  encapsulated as Linux processes?

- architectural simplicity: instead of splitting memory management and scheduling between Xen and domain 0, use just the Linux memory management and scheduler - use standard tools (top(1), kill(1)) and security model (permissions on /dev/kvm)
- much smaller codebase (although paravirtualization is not included (yet))
- no changes to core code
- easy to upgrade an existing system
- easier for drive-by virtualization (modprobe kvm; do-your-stuff; ctrl-C; rmmod kvm) - longer term, better performance since there's no need to switch to domain 0 for I/O (instead just switch to user mode of the VM's process)

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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