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

[Avi Kivity <avi@qumranet.com>]

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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