Re: [Qemu-devel] [PATCH 0/2][RFC] postcopy migration: Linux char device for postcopy

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/02/2012 06:05 PM, Andrea Arcangeli wrote:
> On Thu, Dec 29, 2011 at 06:01:45PM +0200, Avi Kivity wrote:
>> On 12/29/2011 06:00 PM, Avi Kivity wrote:
>>> The NFS client has exactly the same issue, if you mount it with the intr
>>> option.  In fact you could use the NFS client as a trivial umem/cuse
>>> prototype.
>>
>> Actually, NFS can return SIGBUS, it doesn't care about restarting daemons.
>
> During KVMForum I suggested to a few people that it could be done
> entirely in userland with PROT_NONE.

Or MAP_NORESERVE.

Anything you do that is CUSE-based should be doable in a separate QEMU 
thread (rather than a different process that talks to CUSE).  If a 
userspace CUSE-based solution could be done with acceptable performance, 
the same thing would have the same or better performance if done 
entirely within QEMU.

> So the problem is if we do it in
> userland with the current functionality you'll run out of VMAs and
> slowdown performance too much.
>
> But all you need is the ability to map single pages in the address
> space.

Would this also let you set different pgprots for different pages in the 
same VMA?  It would be useful for write barriers in garbage collectors 
(such as boehm-gc).  These do not have _that_ many VMAs, because every 
GC cycles could merge all of them back to a single VMA with PROT_READ 
permissions; however, they still put some strain on the VM subsystem.

Paolo