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.