linux-kernel - Re: [PATCH 1/3] arm64: clean up trampoline vector loads

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Date:   Mon, 23 Mar 2020 12:07:00 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Rémi Denis-Courmont <remi@...lab.net>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, james.morse@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] arm64: clean up trampoline vector loads

On Thu, Mar 19, 2020 at 11:14:05AM +0200, Rémi Denis-Courmont wrote:
> From: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> 
> This switches from custom instruction patterns to the regular large
> memory model sequence with ADRP and LDR. In doing so, the ADD
> instruction can be eliminated in the SDEI handler, and the code no
> longer assumes that the trampoline vectors and the vectors address both
> start on a page boundary.
> 
> Signed-off-by: Rémi Denis-Courmont <remi.denis.courmont@...wei.com>
> ---
>  arch/arm64/kernel/entry.S | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index e5d4e30ee242..24f828739696 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -805,9 +805,9 @@ alternative_else_nop_endif
>  2:
>  	tramp_map_kernel	x30
>  #ifdef CONFIG_RANDOMIZE_BASE
> -	adr	x30, tramp_vectors + PAGE_SIZE
> +	adrp	x30, tramp_vectors + PAGE_SIZE
>  alternative_insn isb, nop, ARM64_WORKAROUND_QCOM_FALKOR_E1003
> -	ldr	x30, [x30]
> +	ldr	x30, [x30, #:lo12:__entry_tramp_data_start]

I think this is busted for !4K kernels once we reduce the alignment of
__entry_tramp_data_start.

The ADRP gives us a 64K aligned address (with bits 15:0 clear). The lo12
relocation gives us bits 11:0, so we haven't accounted for bits 15:12.
I think that's what's causing the hang Catalin sees with 64K pages (and
would also be a problem for 16K pages).

Ideally, we'd account for those bits with the ADRP, but I'm not sure
that an ELF relocation can encode symbol + addr + symbol:15-12, so we
likely nned more instructions to explicitly mask that in.

... either that, or leave this page aligned.

>  #else
>  	ldr	x30, =vectors
>  #endif
> @@ -953,9 +953,8 @@ SYM_CODE_START(__sdei_asm_entry_trampoline)
>  1:	str	x4, [x1, #(SDEI_EVENT_INTREGS + S_ORIG_ADDR_LIMIT)]
>  
>  #ifdef CONFIG_RANDOMIZE_BASE
> -	adr	x4, tramp_vectors + PAGE_SIZE
> -	add	x4, x4, #:lo12:__sdei_asm_trampoline_next_handler
> -	ldr	x4, [x4]
> +	adrp	x4, tramp_vectors + PAGE_SIZE
> +	ldr	x4, [x4, #:lo12:__sdei_asm_trampoline_next_handler]

Likewise here.

Thanks,
Mark.

>  #else
>  	ldr	x4, =__sdei_asm_handler
>  #endif
> -- 
> 2.26.0.rc2
>