Re: [Qemu-devel] Re: [PATCH, RFC] virtio_blk: add cache control support

[Rusty Russell <rusty@rustcorp.com.au>]

On Thu, 17 Mar 2011 15:21:22 +0100, Christoph Hellwig <hch@lst.de> wrote:
> On Thu, Mar 17, 2011 at 03:36:08PM +1030, Rusty Russell wrote:
> > OK, under what circumstances could it fail?
> > 
> > If you're using this mechanism to indicate that the host doesn't support
> > the feature, that's making an assumption about the nature of config
> > space writes which isn't true for non-PCI virtio.
> > 
> > ie. lguest and S/390 don't trap writes to config space.
> > 
> > Or perhaps they should?  But we should be explicit about needing it...
> 
> We have the features flag to indicate if updating the caching mode is
> supported, but we we could still fail it for other reasons - e.g. we could fail
> to reopen the file with/without O_SYNC.

OK, then I think you need to make it a real command and feed it into the
request queue.

The theory behind config space is that it's for advertising, not for
interaction.  And it's not ever very good at that...

>  But if lguest or S/390 don't support
> trapping config space write this feature won't work for them at all.  As do
> other features that make use of config space write, e.g. updating the MAC
> address for virtio-net.

Yes, and that was a mistake.  What does qemu do with a partially-written
MAC address?  Lguest ignores it, what does S/390 do?

Alex?

Cheers,
Rusty.