Re: [Qemu-devel] bs->enable_write_cache and the guest ABI

[Christoph Hellwig <hch@lst.de>]

On Mon, Mar 08, 2010 at 09:47:32AM +0000, Jamie Lokier wrote:
> Assuming the outcome is that it becomes a qdev property, and stays
> preserved across migrations, even if the backing device access
> changes, then I think the right thing is to dynamically decide to set
> O_DSYNC and/or call fdatasync before completing writes from qemu when
> the guest thinks enable_write_cache=0 (or sets it to 0).  With
> cache=none, that would set O_DSYNC|O_DIRECT if the two flags do work
> properly together on our favourite hosts.
> 
> Thus enable_write_cache won't always have the default value for the
> different backing device access type, but it will match the guest's
> expectations and be actually safe.  Moreover more, by responding to
> the guest changing that, it's closer to behaving like real harware.

I'd have to look at other uses of qdev, but I would assume that we
always look at the qdev properties and only use the existing drive
suboptions as compatiblity if we do not have the qdev properties set.