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

[Michael Kelley]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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