linux-kernel - Re: [PATCH 2/5] x86: get rid of KERNEL_STACK

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Message-ID: <20150320162151.GF17058@pd.tnic>
Date:	Fri, 20 Mar 2015 17:21:51 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, 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 2/5] x86: get rid of KERNEL_STACK_OFFSET

On Thu, Mar 19, 2015 at 06:17:46PM +0100, Denys Vlasenko wrote:
> PER_CPU_VAR(kernel_stack) was set up in a way where it points
> five stack slots below the top of stack.
> 
> Presumably, it was done to avoid one "sub $5*8,%rsp"
> in syscall/sysenter code paths, where iret frame needs to be
> created by hand.
> 
> Ironically, none of them benefits from this optimization,
> since all of them need to allocate additional data on stack
> (struct pt_regs), so they still have to perform subtraction.
> 
> This patch eliminates KERNEL_STACK_OFFSET.
> 
> PER_CPU_VAR(kernel_stack) now points directly to top of stack.
> pt_regs allocations are adjusted to allocate iret frame as well.
> Hopefully we can merge it later with 32-bit specific
> PER_CPU_VAR(cpu_current_top_of_stack) variable...
> 
> Net result in generated code is that constants in several insns
> are changed.
> 
> This change is necessary for changing struct pt_regs creation
> in SYSCALL64 code path from MOV to PUSH instructions.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.kernel.org

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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