linux-kernel - Re: [PATCH v2 16/18] arm64: entry: Add fake CPU feature for unmapping the kernel at EL0

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Date:   Fri, 1 Dec 2017 13:55:30 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, ard.biesheuvel@...aro.org,
        sboyd@...eaurora.org, dave.hansen@...ux.intel.com,
        keescook@...omium.org, msalter@...hat.com, labbott@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH v2 16/18] arm64: entry: Add fake CPU feature for
 unmapping the kernel at EL0

On Thu, Nov 30, 2017 at 04:39:44PM +0000, Will Deacon wrote:
> -#ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> -	eret
> -#else
>  	.if	\el == 0
> +alternative_insn eret, nop, ARM64_UNMAP_KERNEL_AT_EL0

Since we patch this eret ...

> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +alternative_if ARM64_UNMAP_KERNEL_AT_EL0
>  	bne	4f
>  	msr	far_el1, x30
>  	tramp_alias	x30, tramp_exit_native
> @@ -334,10 +336,11 @@ alternative_else_nop_endif
>  4:
>  	tramp_alias	x30, tramp_exit_compat
>  	br	x30
> +alternative_else_nop_endif
> +#endif

... we don't need the alternative here. This code won't be executed when
not needed, and the alternative just bloats the kernel.

We can/should keep the ifdef, though.

Thanks,
Mark.