linux-kernel - Re: [PATCH] KVM: arm64: Disable LTO in hyp

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Date:   Thu, 4 Mar 2021 18:38:17 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        James Morse <james.morse@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: arm64: Disable LTO in hyp

On Thu, Mar 4, 2021 at 2:34 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> On Thu, Mar 4, 2021 at 2:17 PM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On Thu, 04 Mar 2021 21:25:41 +0000,
> > Sami Tolvanen <samitolvanen@...gle.com> wrote:
> > >
> > > On Thu, Mar 4, 2021 at 11:15 AM Marc Zyngier <maz@...nel.org> wrote:
> > > >
> > > > On Thu, 04 Mar 2021 18:45:44 +0000,
> > > > Sami Tolvanen <samitolvanen@...gle.com> 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

Note that disabling LTO here has essentially the same effect as using
__section(".text"). It stops the compiler from splitting these
functions into .text.* sections and makes it less likely that
hyp_panic() ends up too far away from __guest_enter().

If neither of these workarounds sound appealing, I suppose we could
alternatively change hyp/entry.S to use adr_l for hyp_panic. Thoughts?

Sami