On Thu, 17 Mar 2011 15:21:22 +0100, Christoph Hellwig <h...@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.
