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

[Ard Biesheuvel]

On Fri, 5 Mar 2021 at 12:38, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Fri, 05 Mar 2021 02:38:17 +0000,
> Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> >
> > On Thu, Mar 4, 2021 at 2:34 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> > >
> > > 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:
>
> [...]
>
> > > > > 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?
>
> That would be an actual fix instead of a workaround, as it would
> remove existing assumptions about the relative locations of the two
> objects. I guess you need to fix both instances with something such
> as:
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index b0afad7a99c6..a43e1f7ee354 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -85,8 +85,10 @@ SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
>
>         // If the hyp context is loaded, go straight to hyp_panic
>         get_loaded_vcpu x0, x1
> -       cbz     x0, hyp_panic
> -
> +       cbnz    x0, 1f
> +       adr_l   x0, hyp_panic
> +       br      x0
> +1:

Agree with replacing the conditional branches that refer to external
symbols: the compiler never emits those, for the reason we are seeing
here, i.e., the range is simply insufficient.

But let's just use 'b hyp_panic' instead, no?



>         // The hyp context is saved so make sure it is restored to allow
>         // hyp_panic to run at hyp and, subsequently, panic to run in the host.
>         // This makes use of __guest_exit to avoid duplication but sets the
> @@ -94,7 +96,7 @@ SYM_INNER_LABEL(__guest_exit_panic, SYM_L_GLOBAL)
>         // current state is saved to the guest context but it will only be
>         // accurate if the guest had been completely restored.
>         adr_this_cpu x0, kvm_hyp_ctxt, x1
> -       adr     x1, hyp_panic
> +       adr_l   x1, hyp_panic
>         str     x1, [x0, #CPU_XREG_OFFSET(30)]
>
>         get_vcpu_ptr    x1, x0
>
> which is completely untested. I wouldn't be surprised if there were
> more of these somewhere.
>

A quick grep gives me

$ objdump -r vmlinux.o |grep BR19
000000000005b6e0 R_AARCH64_CONDBR19  hyp_panic
0000000000418e08 R_AARCH64_CONDBR19  __memcpy
0000000000418e14 R_AARCH64_CONDBR19  __memcpy
0000000000003818 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003898 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003918 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003998 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003a18 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003a98 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003b18 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003b98 R_AARCH64_CONDBR19  __kvm_nvhe___guest_exit_panic
0000000000003c10 R_AARCH64_CONDBR19  __kvm_nvhe___host_exit
0000000000003c1c R_AARCH64_CONDBR19  __kvm_nvhe___host_exit
00000000000064f0 R_AARCH64_CONDBR19  __kvm_nvhe_hyp_panic
000000000000078c R_AARCH64_CONDBR19  __kvm_nvhe___kvm_handle_stub_hvc

So there are definitely a few other cases that require another look.