On 7/21/22 14:27, Daniel P. Berrangé wrote:
Pre-existing guests which have merely upgraded their QEMU instance won't get the feature, because they'll be fixed on the old machine type to guarantee no guest ABI change. This isn't a problem, as such pre-existing guests likely won't have the new Linux code to consume the seed anyway.
In fact this isn't a given either and depends on how these pre-existing guests are managed. People that do not use Libvirt, or that just start their guests with "virsh create" and some on-disk XML file, _will_ get the new feature.
The way QEMU does things is that fw_cfg is part of guest ABI, and this is _not_ up for discussion. You're not the first one to be confused[1] and you probably will not be the last, so this means that the whole concept of guest ABI should be documented better.
(By the way, even though I agree that the failure is completely theoretical apart from the record-replay case, it can actually be reproduced easily by sticking a long-enough sleep in pc-bios/optionrom/linuxboot_dma.c).
Paolo
