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Date: Tue, 16 Jun 2015 14:46:20 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Pavel Machek <pavel@ucw.cz>
CC: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] x86, acpi: Eliminate saved_eip
References: <1434288190-24993-1-git-send-email-dvlasenk@redhat.com> <20150616100546.GA22928@amd>
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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.