Re: [PATCH] KVM: arm64: Disable LTO in hyp

[Sami Tolvanen]

On Thu, Mar 4, 2021 at 2:17 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Thu, 04 Mar 2021 21:25:41 +0000,
> Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> >
> > On Thu, Mar 4, 2021 at 11:15 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > >
> > > On Thu, 04 Mar 2021 18:45:44 +0000,
> > > Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> > > >
> > > > allmodconfig + CONFIG_LTO_CLANG_THIN=y fails to build due to following
> > > > linker errors:
> > > >
> > > >   ld.lld: error: irqbypass.c:(function __guest_enter: .text+0x21CC):
> > >
> > > I assume this message is only an oddity, right? Because
> > > __guest_enter() is as far as you can imagine from irqbypass.c...
> >
> > I'm not sure what's up with the filename in the error message. Fangrui
> > or Nick probably have a better idea.
> >
> > > >   relocation R_AARCH64_CONDBR19 out of range: 2031220 is not in
> > > >   [-1048576, 1048575]; references hyp_panic
> > > >   >>> defined in vmlinux.o
> > > >
> > > >   ld.lld: error: irqbypass.c:(function __guest_enter: .text+0x21E0):
> > > >   relocation R_AARCH64_ADR_PREL_LO21 out of range: 2031200 is not in
> > > >   [-1048576, 1048575]; references hyp_panic
> > > >   >>> defined in vmlinux.o
> > > >
> > > > As LTO is not really necessary for the hypervisor code, disable it for
> > > > the hyp directory to fix the build.
> > >
> > > Can you shed some light on what the problem is exactly?
> >
> > I assume hyp_panic() ends up being placed too far from __guest_enter()
> > when the kernel is large enough. Possibly something to do with LLVM
> > always splitting functions into separate sections with LTO. I'm not
> > sure why the linker cannot shuffle things around to make everyone
> > happy in this case, but I confirmed that this patch also fixes the
> > build issue for me:
> >
> > diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> > index af8e940d0f03..128197b7c794 100644
> > --- a/arch/arm64/kvm/hyp/vhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> > @@ -214,7 +214,7 @@ static void __hyp_call_panic(u64 spsr, u64 elr, u64 par)
> >  }
> >  NOKPROBE_SYMBOL(__hyp_call_panic);
> >
> > -void __noreturn hyp_panic(void)
> > +void __noreturn hyp_panic(void) __section(".text")
> >  {
> >         u64 spsr = read_sysreg_el2(SYS_SPSR);
> >         u64 elr = read_sysreg_el2(SYS_ELR);
> >
>
> We're getting into black-magic territory here. Why wouldn't hyp_panic
> be in the .text section already?

It's not quite black magic. LLVM essentially flips on
-ffunction-sections with LTO and therefore, hyp_panic() will be in
.text.hyp_panic in vmlinux.o, while __guest_enter() will be in .text.
Everything ends up in .text when we link vmlinux, of course.

$ readelf --sections vmlinux.o | grep hyp_panic
  [3936] .text.hyp_panic   PROGBITS         0000000000000000  004b56e4

Sami