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

[Christoph Hellwig]

On Mon, May 16, 2011 at 04:10:21PM -0500, Anthony Liguori wrote:
> 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.

Exactly.  Setting/clearing the BDRV_O_NO_FLUSH also seems useful, maybe
in addition to also allowing an equivalent for the writethrough modes.