linux-kernel - Re: [PATCH v17 05/15] arm64: kexec: skip relocation code for inplace kexec

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Message-ID: <20210929121336.GB21631@willie-the-truck>
Date:   Wed, 29 Sep 2021 13:13:37 +0100
From:   Will Deacon <will@...nel.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, ebiederm@...ssion.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        corbet@....net, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        james.morse@....com, vladimir.murzin@....com,
        matthias.bgg@...il.com, linux-mm@...ck.org, mark.rutland@....com,
        steve.capper@....com, rfontana@...hat.com, tglx@...utronix.de,
        selindag@...il.com, tyhicks@...ux.microsoft.com,
        kernelfans@...il.com, akpm@...ux-foundation.org,
        madvenka@...ux.microsoft.com
Subject: Re: [PATCH v17 05/15] arm64: kexec: skip relocation code for inplace
 kexec

On Thu, Sep 16, 2021 at 07:13:15PM -0400, Pasha Tatashin wrote:
> In case of kdump or when segments are already in place the relocation
> is not needed, therefore the setup of relocation function and call to
> it can be skipped.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> Suggested-by: James Morse <james.morse@....com>
> ---
>  arch/arm64/kernel/machine_kexec.c   | 34 ++++++++++++++++++-----------
>  arch/arm64/kernel/relocate_kernel.S |  3 ---
>  2 files changed, 21 insertions(+), 16 deletions(-)

[...]

> @@ -188,19 +190,25 @@ void machine_kexec(struct kimage *kimage)
>  	local_daif_mask();
>  
>  	/*
> -	 * cpu_soft_restart will shutdown the MMU, disable data caches, then
> -	 * transfer control to the kern_reloc which contains a copy of
> -	 * the arm64_relocate_new_kernel routine.  arm64_relocate_new_kernel
> -	 * uses physical addressing to relocate the new image to its final
> -	 * position and transfers control to the image entry point when the
> -	 * relocation is complete.
> +	 * Both restart and cpu_soft_restart will shutdown the MMU, disable data
> +	 * caches. However, restart will start new kernel or purgatory directly,
> +	 * cpu_soft_restart will transfer control to arm64_relocate_new_kernel
>  	 * In kexec case, kimage->start points to purgatory assuming that
>  	 * kernel entry and dtb address are embedded in purgatory by
>  	 * userspace (kexec-tools).
>  	 * In kexec_file case, the kernel starts directly without purgatory.
>  	 */
> -	cpu_soft_restart(kimage->arch.kern_reloc, kimage->head, kimage->start,
> -			 kimage->arch.dtb_mem);
> +	if (kimage->head & IND_DONE) {
> +		typeof(__cpu_soft_restart) *restart;
> +
> +		cpu_install_idmap();
> +		restart = (void *)__pa_symbol(function_nocfi(__cpu_soft_restart));
> +		restart(is_hyp_nvhe(), kimage->start, kimage->arch.dtb_mem,
> +			0, 0);

Why can't you call:

	cpu_soft_restart(kimage->start, kimage->arch.dtb_mem, 0, 0);

here instead of open-coding it?

Will