From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bombadil.infradead.org ([198.137.202.133]:34620 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727483AbeKIKKj (ORCPT ); Fri, 9 Nov 2018 05:10:39 -0500 Date: Thu, 8 Nov 2018 16:32:25 -0800 From: Matthew Wilcox Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack Message-ID: <20181109003225.GQ3074@bombadil.infradead.org> References: <20181011151523.27101-1-yu-cheng.yu@intel.com> <20181011151523.27101-5-yu-cheng.yu@intel.com> <4295b8f786c10c469870a6d9725749ce75dcdaa2.camel@intel.com> <043a17ef-dc9f-56d2-5fba-1a58b7b0fd4d@intel.com> <20181108220054.GP3074@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Dave Hansen Cc: Andy Lutomirski , Yu-cheng Yu , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , "open list:DOCUMENTATION" , Linux-MM , linux-arch , Linux API , Arnd Bergmann , Balbir Singh , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H. J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , "Shanbhogue, Vedvyas" Message-ID: <20181109003225.BNHnhJg3kAYQ39k7U9ohvKPPsqpcA63G-5CWoUGslO8@z> On Thu, Nov 08, 2018 at 03:35:02PM -0800, Dave Hansen wrote: > On 11/8/18 2:00 PM, Matthew Wilcox wrote: > > struct a { > > char c; > > struct b b; > > }; > > > > we want struct b to start at offset 8, but with __packed, it will start > > at offset 1. > > You're talking about how we want the struct laid out in memory if we > have control over the layout. I'm talking about what happens if > something *else* tells us the layout, like a hardware specification > which is what is in play with the XSAVE instruction dictated layout > that's in question here. > > What I'm concerned about is a structure like this: > > struct foo { > u32 i1; > u64 i2; > }; > > If we leave that to natural alignment, we end up with a 16-byte > structure laid out like this: > > 0-3 i1 > 3-8 alignment gap > 8-15 i2 I know you actually meant: 0-3 i1 4-7 pad 8-15 i2 > Which isn't what we want. We want a 12-byte structure, laid out like this: > > 0-3 i1 > 4-11 i2 > > Which we get with: > > struct foo { > u32 i1; > u64 i2; > } __packed; But we _also_ get pessimised accesses to i1 and i2. Because gcc can't rely on struct foo being aligned to a 4 or even 8 byte boundary (it might be embedded in "struct a" from above). > Now, looking at Yu-cheng's specific example, it doesn't matter. We've > got 64-bit types and natural 64-bit alignment. Without __packed, we > need to look out for natural alignment screwing us up. With __packed, > it just does what it *looks* like it does. The question is whether Yu-cheng's struct is ever embedded in another struct. And if so, what does the hardware do?