linux-kernel - Re: [RFC PATCH 1/7] x86/kexec: Clean up and document register use in relocate_kernel

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Message-ID: <b7282cfd3cf15462cfbc89bab9c09bcb8791b33c.camel@intel.com>
Date: Tue, 5 Nov 2024 10:00:55 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>
CC: "x86@...nel.org" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
	"hpa@...or.com" <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "horms@...nel.org" <horms@...nel.org>,
	"dwmw@...zon.co.uk" <dwmw@...zon.co.uk>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: Re: [RFC PATCH 1/7] x86/kexec: Clean up and document register use in
 relocate_kernel_64.S

On Sun, 2024-11-03 at 05:35 +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Add more comments explaining what each register contains, and save the
> preserve_context flag to a non-clobbered register sooner, to keep things
> simpler.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index e9e88c342f75..c065806884f8 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -100,6 +100,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
>  	movq	%r10, CP_PA_SWAP_PAGE(%r11)
>  	movq	%rdi, CP_PA_BACKUP_PAGES_MAP(%r11)
>  
> +	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
> +	movq	%rcx, %r11
> +

Seems moving this here won't break anything.  From functionality's perspective
there's no need move this here, but fine to me since the move is needed for the
sake of adding the comment (below) to identity_mapped.

>  	/* Switch to the identity mapped page tables */
>  	movq	%r9, %cr3
>  
> @@ -116,6 +119,13 @@ SYM_CODE_END(relocate_kernel)
>  
>  SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>  	UNWIND_HINT_END_OF_STACK
> +	/*
> +	 * %rdi	indirection page
> +	 * %rdx start address
> +	 * %r11 preserve_context
> +	 * %r12 host_mem_enc_active
> +	 */
> +

I think adding this comment is the main purpose of this patch.  Since we have
listed 4 regs in the comment, how about also add an entry for %r13?

Something like:

	%r13 original CR4 when relocate_kernel() is invoked

Yeah this is kinda mentioned in later code but it seems it's more complete if we
also mention %r13 here.

Anyway, I suppose adding this comment is kinda helpful, so:

Acked-by: Kai Huang <kai.huang@...el.com>