Subject: Re: [virtio-dev] [PATCH v3] pci: new configuration layout
On Tue, Sep 24, 2013 at 03:36:42PM +0930, Rusty Russell wrote: > "Michael S. Tsirkin" <email@example.com> writes: > > On Mon, Sep 23, 2013 at 10:57:24AM +0930, Rusty Russell wrote: > >> "Michael S. Tsirkin" <firstname.lastname@example.org> writes: > >> > On Thu, Sep 19, 2013 at 11:53:24AM +0930, Rusty Russell wrote: > >> >> We should also define what "locks in" feature negotiation: that was > >> >> simple for single 32-bit field. Should we define this in a > >> >> transport-independent way, or leave it to the transports? > >> >> > >> >> We could overload the DRIVER status bit (which Linux currently calls too > >> >> early, though moving it would be harmless), or add a new one. > >> > > >> > I'm surprised. > >> > I always read 2.2.1. Device Initialization as > >> > an explicit requirement that DRIVER_OK locks the features, > >> > and that's in a transport-independent section and > >> > works for existing guests. > >> > > >> > And if that's not explicit enough, would the proposed > >> > resolution for VIRTIO-30 make it explicit enough? > >> > >> Linux violates this, as it makes the device live *then* sets DRIVER_OK. > > > > Interesting. > > Actually, e.g. net drivers do this, too: they pre-fill the RX VQs. > > > >> This means we can use the device (and *do*, for virtio_blk partition > >> scanning > > > > What does this refer to? virtblk_getgeo? > > That only seems to poke at config space, not VQs. > > > >>) before finalizing features. > > IIRC add_disk() does partition scanning. > > >> We could become compliant by setting DRIVER_OK before calling the > >> driver, but that's be misleading: we'd not expect DRIVER_OK if the > >> driver init failed. > >> So I think we will need a new bit, FEATURES_OK? > >> > >> Cheers, > >> Rusty. > > > > But what's the meaning of DRIVER_OK then? > > It's informational, telling us how far the device got before it failed. > > DRIVER_OK basically indicates that as far as guest is concerned, device > is live. > > > Maybe it means "device can use VQs"? > > > > And if device does not use VQs, does it matter that > > driver can add buffers there? > > If not maybe we can let driver play with features all > > it wants. > > That doesn't really work with the Linux model. And if a future feature > changed ring layout, we'd *really* need to know whether the guest > understood that before it touched the vq. OK so I added DEVICE_OK. Could you look at the latest draft? > > Also all this is really part of discussion for VIRTIO-30 I think? > > It's not really related to the layout - the issue is there > > in old layout as well. > > Correct? > > Yes, except that it was easier to work around when a single write > updated all the features. Still I've opened a new issue... > > Thanks, > Rusty. Hmm you don't think VIRTIO-30 is a good place to track it all?