linux-kernel - Re: [PATCH 1/3] x86, acpi: Eliminate saved

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Date:	Wed, 17 Jun 2015 11:28:32 +0200
From:	Pavel Machek <pavel@....cz>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86, acpi: Eliminate saved_eip

On Tue 2015-06-16 14:46:20, Denys Vlasenko wrote:
> On 06/16/2015 12:05 PM, Pavel Machek wrote:
> > 
> >> diff --git a/arch/x86/kernel/acpi/wakeup_32.S b/arch/x86/kernel/acpi/wakeup_32.S
> >> index 665c6b7..eef2bd3 100644
> >> --- a/arch/x86/kernel/acpi/wakeup_32.S
> >> +++ b/arch/x86/kernel/acpi/wakeup_32.S
> >> @@ -31,17 +31,11 @@ wakeup_pmode_return:
> >>  
> >>  	movl	%cs:saved_magic, %eax
> >>  	cmpl	$0x12345678, %eax
> >> -	jne	bogus_magic
> >> -
> >> -	# jump to place where we left off
> >> -	movl	saved_eip, %eax
> >> -	jmp	*%eax
> >> -
> >> +	je	ret_point
> >>  bogus_magic:
> > 
> > Longjump is supposed to flush the prefetch, but hopefully nothing
> > depends on that detail... ...
> > 
> > no.
> > 
> > https://en.wikibooks.org/wiki/X86_Assembly/Protected_Mode#Entering_Protected_Mode
> > 
> > longjump is supposed to be there. Sorry about that. I guess indirect
> > jump is not neccessary, and we should add a comment there.
> 
> What "long jump"? It's not a far (segment:offset) jump, it's a near
> jump (one which only changes the offset), indirect one through a register.

Yup, you are right, the ljmp is few instructions above that.

> And we aren't switching to or from protected mode here.

wakeup_pmode_return... that's just after switching to protected mode
AFAICT.

Anyway,

Acked-by: Pavel Machek <pavel@....cz>

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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