On 24 June 2013 13:26, Christopher Covington <[email protected]> wrote: > On 06/22/2013 06:51 AM, Peter Maydell wrote: >> On 21 June 2013 19:45, Christopher Covington <[email protected]> wrote: >>> You were proposing to use a valid/existing MagicValue/Version/VendorID with >>> a >>> special DeviceID that does nothing. I'm saying why not use a valid/existing >>> MagicValue/Version/VendorID/DeviceID with a special parameter setting, >>> size=0, >>> that does nothing? > > [...] > >> Also, it's mixing a detail of the backend layer (what >> a zero-sized disk happens to look like) with the transport layer, >> which seems a bit ugly spec-wise. > > I don't think that has to be the case. From what I understand of your > architecture, the device layer is completely opaque to the transport layer, > and the transport layer is immutable, but surely the device layer will at > least know how many transports are available? As long as that's true, can't > the device layer just create real devices and hook them up to transports, and > then create no-op devices and hook them up to any remaining transports?
No, the device creation code just creates all the devices the user asks for. At the point where it can't find a transport to plug one into, the device-creation fails with an error message. It doesn't attempt to find buses with nothing plugged in (since for instance it's entirely reasonable to have a PCI slot with no card in it). >> (Implementation wise I'm not crazy about it either since it would >> be way more complicated than saying "no backend? OK, RAZ/WI".) > > (I thought virtio block devices were already implemented.) Yes, but "what does a transport do if there's no backend" is transport code, whereas the block device is backend code. I actually think the creation of a spurious /dev/vda makes 'zero-sized block backend' a non-starter anyway, though. It's also neither conceptually pure nor pragmatically easy, so it falls between two stools. I could see the rationale for a 'cleanliness of spec' position that we should handle this with a 'null' backend with a new DeviceID which is completely functional (including all the host feature bitmaps, setting up queues, interrupts, etc) but just doesn't do anything. That would be more complicated to implement than RAZ/WI, but not impossible. thanks -- PMM _______________________________________________ Virtualization mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
