Re: [PATCH 10/11] KVM: arm64: nv: Honor guest hypervisor's FP/SVE traps in CPTR_EL2

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Jun 04, 2024 at 12:14:42PM +0100, Marc Zyngier wrote:
> On Mon, 03 Jun 2024 18:28:56 +0100, Oliver Upton <oliver.upton@linux.dev> wrote:
> > Anyway, my _slight_ preference is towards keeping what I have if
> > possible, with a giant comment explaining the reasoning behind it. But I
> > can take your approach instead too.
> 
> I think the only arguments for my own solution are:
> 
> - slightly better codegen (no function call or inlining), and a
>   smaller .text section in switch.o, because the helpers are not
>   cheap:
> 
>   LLVM:
> 
> 	0 .text         00003ef8 (guest_hyp_*_traps_enabled)
> 	0 .text         00003d48 (bit ops)
> 
>   GCC:
> 	0 .text         00002624 (guest_hyp_*_traps_enabled)
> 	0 .text         000024b4 (bit ops)
> 

Oh, that's spectacular :-)

>   Yes, LLVM is an absolute pig because of BTI...
> 
> - tracking the guest's bits more precisely may make it easier to debug
> 
> but these are pretty weak arguments, and I don't really care either
> way at this precise moment.

Yeah, so I think the right direction here is to combine our approaches,
and do direct bit manipulation, but only on bit[0]. That way we still
have an opportunity to document the very intentional simplification of
trap state too.

-- 
Thanks,
Oliver