Laszlo Ersek <lersek@xxxxxxxxxx> writes: > Hi Markus, > > On 02/07/19 10:30, Markus Armbruster wrote: >> The thread got long, let me try to summarize, and elaborate a few >> points. >> >> * The problem at hand is configuring firmware residing in flash memory >> (OVMF requires this) without legacy -drive. >> >> * The wider problem is configuring onboard devices. Our general device >> configuration interface doesn't cover them. Instead, we have a zoo of >> ad hoc interfaces that are much more limited. Some of them we'd >> rather deprecate (-drive, -net), but can't until we have a suitable >> replacements. >> >> I think a board should be a composite object that exposes properties >> of its own and its parts, just like other composite devices, so that >> "create, set properties, realize" just works. That would extend our >> common device configuration mechanism naturally to onboard devices. >> >> A PC board's flash memory device would be just another part. It could >> be something like /machine/q35/cfi.pflash01/ in the QOM tree. To >> configure it, you'd set its properties, such as >> /machine/q35/cfi.pflash01/drive. >> >> Note that this requires a way to set an existing device's properties. >> Perhaps qom-set already works. >> >> * While I do believe we should tackle the wider problem, I'd rather not >> sit on the narrow problem until we crack it. So, what can we do about >> it? >> >> - Paolo proposed to add block backend properties to the PC machine, >> settable like -machine pflash0=BLOCK-BACKEND. >> >> Possible drawback: if we add /machine/q35/pflash0 to the QOM tree >> now, and later replace it by /machine/q35/cfi.pflash01/drive, we'll >> have to deal with yet another machine type variation. We'll live. >> >> - I proposed to sidestep our onboard device configuration problem by >> adding the cfi.pflash01 devices with our existing general device >> configuration interface: -device. Possible since the onboard >> cfi.pflash01 devices are optional. Requires a small extension to >> the firmware descriptors, and a bit of extra work in libvirt to >> process that extension. I think it's workable, but Paolo's idea is >> simpler. >> >> I can give Paolo's idea a try. Objections? >> >> * A flash device supporting multiple regions is desirable, because it's >> what physical hardware has. We currently use multiple flash devices >> instead. We'll be stuck with them for existing machine types due to >> guest ABI and migration compatibility. >> >> * cfi.pflash01 currently requires users to opt out of "bad, do not use". >> It should require opt in, to guard against accidental new uses of >> "bad". >> >> >> PS: Big thanks to László, whose patient guidance helped me map this part >> of the jungle. >> > > I've read the above carefully. > > At the QEMU design level, I don't have any opinion or preference; there > I simply don't know enough -- and don't suffer from bad decisions enough > -- to make sensible comments. > > Regarding the choice betwen "-machine pflash0=BLOCK-BACKEND" and > "-device pflash": I don't object to exploring the former first. > > I'd just like to note that "-machine pflash0=BLOCK-BACKEND" will also > require changes to the firmware descriptor schema. Not to the types that > the schema defines -- and therefore concrete descriptor *documents* that > already conform to the schema wouldn't be affected --, but to the > documentation that the schema directs at management applications. > > The schema is supposed to specify (in the documentation) QEMU command > line options for management applications. If we add "-machine > pflash0=BLOCK-BACKEND", then even if the types in the schema stay the > same, some mappings to the QEMU cmdline will have to be re-documented. Good point. > Of course, that's still easier / less intrusive than changing the types! > ... Which does make me prefer "-machine pflash0=BLOCK-BACKEND", if I'm > being honest. > > (I hope my followup isn't totally useless. I certainly didn't want to > ignore your summary.) Pointing out the need to update these comments is anything but useless. Thanks! -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
