Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT)

[Anthony Liguori]

Jamie Lokier wrote:
> Avi Kivity wrote:
>   
> > Here's a summary of the use cases I saw so far:
> >
> > - casual use, no critical data: write back cache
> >
> > - backing file shared among many guests: read-only, cached
> >
> > - desktop system, but don't lose my data: O_SYNC
> > (significant resources on the host)
> >
> > - dedicated virtualization engine: O_DIRECT
> > (most host resources assigned to guests)
> >     
>
> Sounds alright, but on _my_ desktop system (a laptop), I would use O_DIRECT.
>
> There isn't enough RAM in my system to be happy duplicating data in
> guests and hosts at the same time.  VMs are quite demanding on RAM.
>
> However, if you find a way to map host cached pages into the guest
> without copying - so sharing the RAM - that would be excellent.  It
> can be done in principle, by remapping pages to satisfy IDE/SCSI DMA
> requests.  I don't know if it would be fast enough.  Perhaps it would
> work better in KVM than QEMU.
>   

Actually, this is precisely what I'd like to do.  The key is to 
mmap(MAP_PRIVATE) from the underling file directly into the guest's 
memory.  Should be just as applicable to QEMU as KVM (although for KVM 
we need mmu-notifiers first).

Should be a pretty good win when running multiple guests with the same 
backing file too.

Regards,

Anthony Liguori

> -- Jamie
>