Re: [Qemu-devel] [RFC Patch 0/3]Qemu: Enable dynamic cache change through qemu monitor

[Anthony Liguori]

On 05/16/2011 03:23 PM, Christoph Hellwig wrote:
> Why are you even trying this again?  As explained very clearly last time you
> can't change from a writeback-style to a write-through style I/O from
> the monitor without creating massive data integrity problems.

To further clarify:

Today cache=none|writethrough|writeback does two things.  It:

1) Changes the WCE flag that's visible to the guest

2) Determines whether the host page cache is used for doing guest I/O

As Christoph is very correct in pointing out, we cannot change (1) at 
run time because this is guest visible.  You will break a guest if you 
do this.

But it's still desirable to be able to change (2) at run time.  Before 
we can do this properly though, we need to separate out the logic for 
setting (1) vs. (2).

And ideally, we would allow (1) to be changed by the guest itself at run 
time which allows for full dynamic control.  This is what he's referring 
to below.

Regards,

Anthony Liguori

  See my
> patchset that allows changing this from the guest for how it should be
> done - I just need to get back and revisit the virtio protocol support
> for it.