From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752283AbbCYAMM (ORCPT ); Tue, 24 Mar 2015 20:12:12 -0400 Received: from mga11.intel.com ([192.55.52.93]:14308 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751874AbbCYAML (ORCPT ); Tue, 24 Mar 2015 20:12:11 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.11,461,1422950400"; d="scan'208";a="472136633" Message-ID: <5511FD59.3040503@intel.com> Date: Tue, 24 Mar 2015 17:12:09 -0700 From: Dave Hansen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0 MIME-Version: 1.0 To: Andy Lutomirski CC: Thomas Gleixner , "linux-kernel@vger.kernel.org" , X86 ML , Dave Hansen , Rik van Riel , Suresh Siddha , Ingo Molnar , "H. Peter Anvin" , Fenghua Yu , Oleg Nesterov Subject: Re: [PATCH 01/17] x86, fpu: wrap get_xsave_addr() to make it safer References: <1427235664-25318-1-git-send-email-dave.hansen@intel.com> <1427235664-25318-2-git-send-email-dave.hansen@intel.com> <5511F65A.5020505@intel.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 03/24/2015 04:52 PM, Andy Lutomirski wrote: > On Tue, Mar 24, 2015 at 4:42 PM, Dave Hansen wrote: >> On 03/24/2015 03:28 PM, Andy Lutomirski wrote: >>> Your function appears to be getting it for write (I assume that's what >>> the unlazy_fpu is for), so I'd rather have it called >>> tsk_get_xsave_field_for_write or something like that. >> >> It should be entirely read-only. >> >> For MPX (the only user of get_xsave_addr() iirc), we are only worried >> about getting the status codes (and addresses) out of the bndstatus >> register and making sure that the kernel-recorded bounds directory >> address matches the bndcfgu (configuration) register. >> >> We don't ever write to the registers. > > So why are you unlazying it? Oleg actually suggested it. > IIUC, the xstae for current can be in one of three logical states: > > 1. Live in CPU regs. The in-memory copy is garbage and the state is > in CPU regs. > 2. Lazy. The in-memory copy and the CPU regs match. Writing to > either copy is illegal. > 3. In memory only. Writing to the in-memory copy is safe. > > IIUC, you want to read the xstate, do you're okay with #2 or #3. This > would be tsk_get_xsave_field_for_read in my terminology. > > If you want to write the xstate, you'd need to be in state #3, which > would be tsk_get_xsave_field_for_write. > > IIUC, unlazy_fpu just moves from from state 2 to 3. I won't completely claim to understand what's going on with the FPU code, but I think your analysis is a bit off. unlazy_fpu() does __save_init_fpu() which (among other things) calls xsave to dump the CPU registers to memory. That doesn't make any sense to do if "The in-memory copy and the CPU regs match." IOW, unlazy_fpu() is called when the in-memory copy is garbage and takes us to a state where we can look at the in-memory copy.