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

On Thu, Mar 4, 2021 at 2:34 PM Sami Tolvanen [off-list ref] wrote:

On Thu, Mar 4, 2021 at 2:17 PM Marc Zyngier [off-list ref] wrote:

quoted

On Thu, 04 Mar 2021 21:25:41 +0000,
Sami Tolvanen [off-list ref] wrote:

quoted

On Thu, Mar 4, 2021 at 11:15 AM Marc Zyngier [off-list ref] wrote:

quoted

On Thu, 04 Mar 2021 18:45:44 +0000,
Sami Tolvanen [off-list ref] wrote:

quoted

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.

quoted

quoted

  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

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