Re: comparisons with VMware and Xen

[Anthony Liguori <anthony@codemonkey.ws>]

Sukanto Ghosh wrote:
> 1. Is the maximum no. of  vcpus in a particular guest limited to the
> no. of host cpus ? (I guess not)
>   

No.

> 2. Is there any attempt made to co-schedule all of a guest's vcpus, in
> order to avoid any spinlock holding problem ?
>   

Right now, no.  There is some discussion of gang scheduling within Linux 
but I don't know that it's going anywhere.  Recently, Jeremy 
Fitzhardinge posted some paravirtual spinlock patches for Linux that 
would at least allow for spinlock yielding.

> 3. Are there any means to do content-based page sharing between guests
> as VMware does ?
>   

Yes, KSM.

> 4. Does kvm makes any attempt to avoid TLB flushes while vm-exits and
> vm-entries ? like Xen makes a memory hole ?
> (my guess is that it doesn't needs to as kvm is mapped into a guest's
> address-space and the pages are protected with the help of linux vm.
> Am i right ? )
>   

A TLB flush is mandatory when using hardware virtualization support 
(even with Xen--you are thinking of Xen PV).  However, both Intel and 
AMD now support hardware TLB tagging which reduces the pain of this TLB 
flush.

> Slightly different ones,
>
> 5. How much useful is a balloon driver for kvm, which doesn't makes
> any hard partitions of available physical memory between the guests ?
> Shouldn't the linux VM's knowledge be superior in this case than the
> guest-vm's  ?
>   

Ballooning can be very useful when doing very large changes to the 
amount of guest memory.

> 6. What is coalesced mmio ?
>   

There are a number of times when a large number of MMIO writes occur 
back-to-back.  Think VGA planar updates, writes of a network packet to 
on-chip memory, etc.  Instead of passing each of these writes to 
userspace, we buffer a certain number of them and send a good bit of 
them down to QEMU.  The real win from this is making N-1 of the buffered 
writes into light-weight exits instead of heavy-weight exits.

Regards,

Anthony Liguori

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