RE: [PATCH v11 2/2] Drivers: hv: Introduce mshv_vtl driver

Michael Kelley
2025-11-12T09:44:23Z

From: Peter Zijlstra <peterz@infradead.org> 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