On Tue, Jan 20, 2015 at 05:18:23PM +0000, Pawel Moll wrote: > On Mon, 2015-01-19 at 18:36 +0000, Michael S. Tsirkin wrote: > > > > > Well, not quite: as of now I've got legacy block device driver happily > > > > > working on top of compliant (so v2 in MMIO speech) MMIO device - the > > > > > transport if completely transparent here. > > > > > > > > Spec says explicitly it's an illegal configuration. > > > > > > What part of the spec exactly? The closest I can think of are 2.2.3, 6.2 > > > and 6.3. Both legacy and spec-complaint MMIO transports will satisfy all > > > requirements from those paragraphs, whether VIRTIO_F_VERSION_1 is > > > negotiated or not. > > > > The parts that say VIRTIO_F_VERSION_1 MUST be set. > > I'll look up the chapter and verse later if you like. > > That would be: > > "6.1 Driver Requirements: Reserved Feature Bits > A driver MUST accept VIRTIO_F_VERSION_1 if it is offered. A driver MAY > fail to operate further if VIRTIO_F_VERSION_1 is not offered." > > "6.2 Device Requirements: Reserved Feature Bits > A device MUST offer VIRTIO_F_VERSION_1. A device MAY fail to operate > further if VIRTIO_F_VERSION_1 is not accepted." That, and there are a bunch of other changes we made in devices as compared to virtio 0.9. > > Both are perfectly clear and reasonable for me. And the MMIO transport > in both versions (pre-spec and the spec one) will meet those > requirements, as it was able to pass more than 32 features bits from the > very beginning. Good. But your current code will also try to support a mix: device that uses the new transport underneath, but old layout and semantics on top. This has no value: the spec does *not* define such devices and it also does not match any hosts, so this just adds to support matrix. And if linux supports it, someone *will* ship such a device and we'll be stuck with it forever. So please, detect out of spec devices and fail gracefully. pci and s390 do this already. > > Can you please also add a comment? > > E.g. > > /* > > * MMIO uses ID 0 to mean device not present. Avoid probing > > * the device further: it's sure to fail anyway. > > */ > > There is a comment: > > > On Fri, 2014-12-19 at 18:38 +0000, Pawel Moll wrote: > > > + /* > > > + * ID 0 means a dummy (placeholder) device, skip quietly > > > + * (as in: no error) with no further actions > > > + */ > > > But I'm can use your wording if you find it better. > > Pawel Either that or combine them in some way. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
