Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754068AbaBAT15 (ORCPT ); Sat, 1 Feb 2014 14:27:57 -0500 Received: from mail-vb0-f42.google.com ([209.85.212.42]:39432 "EHLO mail-vb0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752886AbaBAT1z (ORCPT ); Sat, 1 Feb 2014 14:27:55 -0500 MIME-Version: 1.0 In-Reply-To: References: Date: Sat, 1 Feb 2014 11:27:54 -0800 X-Google-Sender-Auth: _QfGvafHXnByc8UZOWPmPsr-Js8 Message-ID: Subject: Re: [PATCH] Make math_state_restore() save and restore the interrupt flag From: Linus Torvalds To: Suresh Siddha Cc: Nate Eldredge , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "the arch/x86 maintainers" , stable , Linux Kernel Mailing List , Maarten Baert , Jan Kara , George Spelvin , Pekka Riikonen Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 30, 2014 at 11:33 PM, Suresh Siddha wrote: > > a. delayed dynamic allocation of FPU state area was not a good idea > (from me). Given most of the future cases will be anyway using eager > FPU (because of processor features like xsaveopt etc, applications > implicitly using FPU because of optimizations in commonly used > libraries etc), we should probably go back to allocation of FPU state > area during thread creation for everyone (including non-eager cases). Yes, I suspect that will help some, and probably fix this particular bug. That said, regardless of the allocation issue, I do think that it's stupid for kernel_fpu_{begin,end} to save the math state if "used_math" was not set. So I do think__kernel_fpu_end() as-s is buggy and stupid. So I do think we should *either* say (a) "we don't want to restore at all, because once the kernel starts using math, it might do so a lot, and saving/restoring is a bad idea": void __kernel_fpu_end(void) { stts(); } *or* (b) make the use_eager_fpu() case check tsk_used_math() (in which case we had better already have an allocation!) void __kernel_fpu_end(void) { if (use_eager_fpu()) { struct task_struct *me = current; if (tsk_used_math(me) && likely(!restore_fpu_checking(me))) return; } stts(); } Quite frankly, I'd almost lean towards (a). Comments? Does anybody have any loads where the kernel does a lot of fpu stuff (ie network encryption using the hw engines or something)? I'd really like to hear if it makes a difference.. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/