From: Peter Zijlstra <[email protected]> Sent: Wednesday, November 12, 2025 1:37 AM > > On Wed, Nov 12, 2025 at 04:12:08AM +0000, Michael Kelley wrote: > > > > @@ -96,3 +97,10 @@ SYM_FUNC_START(__mshv_vtl_return_call) > > > pop %rbp > > > RET > > > SYM_FUNC_END(__mshv_vtl_return_call) > > > + > > > + .section .discard.addressable,"aw" > > > + .align 8 > > > + .type > > > __UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0, @object > > > + .size > > > __UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0, 8 > > > +__UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0: > > > + .quad __SCK____mshv_vtl_return_hypercall > > > > This is pretty yucky itself. > > Definitely doesn't win any prizes, for sure. > > > Why is it better than calling out to a C function? > > It keeps all the code in one place is a strong argument. > > > Is it because in spite of the annotations, there's no guarantee the C > > compiler won't generate some code that messes up a register value? Or is > > there some other reason? > > There is that too, a frame pointer build would be in its right to add a > stack frame (although they typically won't in this case). And the C ABI > doesn't provide the guarantees your need, so calling out into C is very > much you get to keep the pieces. > > > Does the magic "_662.0" have any significance? Or is it just some > > uniqueness salt on the symbol name? > > Like Paolo already said, that's just the crazy generated by our > __ADRESSABLE() macro, this name is mostly irrelevant, all we really need > is a reference to that __SCK____mshv_vtl_return_hypercall symbol so it > ends up in the symbol table. (And the final link will then complain if > the symbol doesn't end up being resolved) > > Keeping the name somewhat in line with __ADDRESSABLE() has the advantage > that you can clearly see where it comes from, but yeah, we can strip of > the number if you like.
Thanks. If that symbol is referenced only by these few lines, I'd go with something even shorter and simpler. Perhaps: .section .discard.addressable,"aw" .align 8 .type vtl_return_sym, @object .size vtl_return_sym, 8 vtl_return_sym: .quad __SCK____mshv_vtl_return_hypercall Regardless of the choice of symbol name, add a comment about mimicking __ADDRESSABLE(). That feels less messy to me, but it's Naman's call. Michael
