On Mon, Oct 04, 2021 at 04:50:51PM +0200, Laszlo Ersek wrote: > On 10/04/21 11:59, Richard W.M. Jones wrote: > > It turns out that changing the qemu implementation is painful, > > particularly if we wish to maintain backwards compatibility of the > > command line and live migration. > > > > Instead I opted to document comprehensively what all the > > different hypervisors do: > > > > https://github.com/libguestfs/virt-v2v/blob/master/docs/vm-generation-id-across-hypervisors.txt > > > Unfortunately QEMU made a significant mistake when implementing this > > feature. Because the string is 128 bits wrong, they decided it must > ^^^^^^^^^^^^^^ > > Haha, that's a great typo :) > > > be a UUID (as you can see above there is no evidence that Microsoft > > who wrote the original spec thought it was). Following from this > > incorrect assumption, they stated that the "UUID" must be supplied to > > qemu in big endian format and must be byteswapped when writing it to > > guest memory. Their documentation says that they only do this for > > little endian guests, but this is not true of their implementation > > which byte swaps it for all guests. > > I don't think this is what section "Endian-ness Considerations" in > "docs/specs/vmgenid.txt" says. That text says that the *device* uses > little-endian format. That's independent of the endianness of *CPU* of > the particular guest architecture. > > So the byte-swapping in the QEMU code occurs unconditionally because > QEMU's UUID type is inherently big endian, and the device model in > question is fixed little endian. The guest CPU's endianness is > irrelevant as far as the device is concerned. > > If a BE guest CPU intends to read the generation ID and to interpret it > as a set of integers, then the guest CPU is supposed to byte-swap the > appropriate fields itself. > > > References > > I suggest adding two links in this section, namely to: > - docs/specs/vmgenid.txt > - hw/acpi/vmgenid.c Fair enough - I've made the changes you suggest (same URL as above). Thanks, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
