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

[supriya kannery]

Anthony Liguori wrote:
> 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.
ok
> 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).
Will go through the code in detail to understand handling of (1) and (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.