linux-kernel - Re: [PATCH 1/2] x86/kexec: Fix a comment of swap

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Message-ID: <4c35dfdadff667ae5ea00dd6002b8cd73301dc22.camel@intel.com>
Date: Tue, 20 Aug 2024 10:32:39 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC: "Hansen, Dave" <dave.hansen@...el.com>, "bp@...en8.de" <bp@...en8.de>,
	"x86@...nel.org" <x86@...nel.org>, "peterz@...radead.org"
	<peterz@...radead.org>, "hpa@...or.com" <hpa@...or.com>, "mingo@...hat.com"
	<mingo@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nik.borisov@...e.com" <nik.borisov@...e.com>
Subject: Re: [PATCH 1/2] x86/kexec: Fix a comment of swap_pages() assembly

On Tue, 2024-08-20 at 11:40 +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 20, 2024 at 12:21:11AM +1200, Kai Huang wrote:
> > When relocate_kernel() gets called, %rdi holds 'indirection_page' and
> > %rsi holds 'page_list'.  And %rdi always holds 'indirection_page' when
> > swap_pages() is called.
> > 
> > Therefore the comment of the first line code of swap_pages()
> > 
> > 	movq    %rdi, %rcx      /* Put the page_list in %rcx */
> > 
> > .. isn't correct because it actually moves the 'indirection_page' to
> > the %rcx.  Fix it.
> > 
> > Signed-off-by: Kai Huang <kai.huang@...el.com>
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> Looks like it got broken by 4bfaaef01a1b ("[PATCH] Avoid overwriting the
> current pgd (V4, x86_64)")
> 

Thanks for finding this.  I spent some time yesterday trying to do so but it
wasn't obvious to me.  :-)

Yes that line was firstly introduced by commit 5234f5eb04abb ("[PATCH] kexec:
x86_64 kexec implementation") but by that time it was correct: %rdi indeed
holds 'page_list'.  The commit you mentioned above adds a new (first) argument
to relocate_kernel() and %rdi was changed to hold 'indirection_page', but the
comment was leftover.

But the two commits were introduced at 2004 and 2006, so I don't think it
worth any Fixes tag?