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

From: Rusty Russell
Date: Wed Mar 23 2011 - 20:23:35 EST

On Thu, 17 Mar 2011 15:21:22 +0100, Christoph Hellwig <hch@xxxxxx> 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?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at