We are working with Hyper-V team on changing the structure definition for the
hypercall HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST_EX and corresponding protocol and internal handling in Hyper-V.
Once we reach the agreement, it could be changed on both Linux and FreeBSD guest.

Also, this is not a bug, but specific to particular compiler. As we defined same structure in Linux and it works without adding any additional flag. There is no warning during compiling time on Linux. I think FreeBSD uses clang which is stricter than the compilers other OSes use. Second, we have expressed the concerns we got from FreeBSD community about using this structure to the team who defined HyperV protocol in Microsoft. It is possible they may change it in the future protocol. For the time being to improve the tlb flush performance, we decide to still use this structure for the current patch.

Linux's gcc has this bug (or rather extension) that allows this non-standards-conforming construct to have a well-defined meaning. When using non-standards-conforming constructs, that's a bug, regardless of whether or not other compilers don't complain. The fact there's no warning on Linux carries no weight for me in this argument.