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
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?
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