linux-kernel - Re: [PATCHv2 1/4] x86_64: cleanup THREAD

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Date:	Wed, 21 Jan 2015 17:20:09 +0100
From:	Alexander van Heukelum <heukelum@...tmail.fm>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>, Borislav Petkov <bp@...e.de>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCHv2 1/4] x86_64: cleanup THREAD_INFO(reg,offset) macro

On Wed, Jan 21, 2015, at 14:40, Denys Vlasenko wrote:
> On Sun, Jan 18, 2015 at 12:45 PM, Alexander van Heukelum
> <heukelum@...tmail.fm> wrote:
> > The macro THREAD_INFO(reg,offset) is used in assembly to compute the
> > offset between the user ptregs and the thread_info struct. Change
> > the macro and all its uses so that offset is given as the current
> > top of stack in the pt_regs frame. The generated code is identical.
> 
> What is the purpose of doing this?
> 
> I don't mean to say that it's pointless, but it is also not obvious
> why it's better than the code before patch.

Just because I had a hard time understanding what the offset
parameter means. Turns out Borislav did another variant in his
patchset. Quoting him from the commit message which made
that the change:
> Semi-mysterious expressions THREAD_INFO(%rsp,RIP) - "why RIP??"
> are now replaced by more logical THREAD_INFO(%rsp,SIZEOF_PTREGS) -
> "rsp is SIZEOF_PTREGS bytes below the top, calculate
> thread_info's address using that information"

Borislav expresses "offset" as an offset from the stack's page
boundary,while I chose the offset from the start of struct
pt_regs on the stack.
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