Re: [PATCH] KVM: arm64: Don't use cbz/adr with external symbols

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Mar 05, 2021 at 12:21:24PM -0800, Sami Tolvanen 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):
>   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
> 
> This is because with LTO, the compiler ends up placing hyp_panic()
> more than 1MB away from __guest_enter(). Use an unconditional branch
> and adr_l instead to fix the issue.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1317
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

I booted an kernel with this patch in it on my Raspberry Pi 4 and booted
a kernel under KVM without any issues.

Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/arm64/kvm/hyp/entry.S | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index b0afad7a99c6..c62265951467 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
> +	b	hyp_panic
>  
> +1:
>  	// 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
> 
> base-commit: 280d542f6ffac0e6d65dc267f92191d509b13b64
> -- 
> 2.30.1.766.gb4fecdf3b7-goog
> 
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