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

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Date:	Tue, 16 Jun 2015 14:46:20 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Pavel Machek <pavel@....cz>
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 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.

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