From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752460AbbCSP2L (ORCPT ); Thu, 19 Mar 2015 11:28:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36015 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751303AbbCSP2I (ORCPT ); Thu, 19 Mar 2015 11:28:08 -0400 Message-ID: <550AEB03.6010701@redhat.com> Date: Thu, 19 Mar 2015 16:28:03 +0100 From: Denys Vlasenko User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Andy Lutomirski CC: Linus Torvalds , Steven Rostedt , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , X86 ML , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 1/3] x86: get rid of KERNEL_STACK_OFFSET References: <1426708056-2785-1-git-send-email-dvlasenk@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/18/2015 09:54 PM, Andy Lutomirski wrote: > On Wed, Mar 18, 2015 at 12:47 PM, 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... >> >> Semi-mysterious expressions THREAD_INFO(%rsp,RIP) - "why RIP??" >> are now replaced by more logical THREAD_INFO(%rsp,SIZEOF_PTREGS) - >> "calculate thread_info's address using information that >> rsp is SIZEOF_PTREGS bytes below the stack top". >> >> 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. >> > > Would it be reasonable to break this up into two pieces: first, remove > KERNEL_STACK_OFFSET from THREAD_INFO and related macros (i.e. make > them relative to current_top_of_stack instead) PER_CPU(cpu_current_top_of_stack) exists only in 32 bits. Can't use it in 64-bit code. PER_CPU(cpu_current_top_of_stack) becomes equal to PER_CPU(kernel_stack) only after this patch. I plan to clean up PER_CPU(cpu_current_top_of_stack)/ PER_CPU(kernel_stack)/PER_CPU(tss->sp0) mess on top of my patches. First: this seems to be the easier way. Second (why do this *after* this patch set, not in it?): the removal of KERNEL_STACK_OFFSET has no strong reason behind it unless we do MOV -> PUSH conversion - in which case it is mandatory: we must have RSP start at top of stack in 64-bit mode. I want to post a minimal patch set which results in PUSH conversion. Want to avoid polluting it with tangential goal of removing duplicated percpu variable.