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Date: Fri, 9 Nov 2018 00:31:26 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>, X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Balbir Singh <bsingharora@gmail.com>, Dave Hansen <dave.hansen@linux.intel.com>, Eugene Syromiatnikov <esyr@redhat.com>, Florian Weimer <fweimer@redhat.com>, "H. J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>, Kees Cook <keescook@chromium.org>, Mike Kravetz <mike.kravetz@oracle.com>, Nadav Amit <nadav.amit@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>, Randy Dunlap <rdunlap@infradead.org>, "Ravi V. Shankar" <ravi.v.shankar@intel.com>, "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>

On Thu, Nov 08, 2018 at 01:22:54PM -0800, Andy Lutomirski wrote:
> > >
> > > Why are these __packed?  It seems like it'll generate bad code for no
> > > obvious purpose.
> >
> > That prevents any possibility that the compiler will insert padding, although in
> > 64-bit kernel this should not happen to either struct.  Also all xstate
> > components here are packed.
> >
> 
> They both seem like bugs, perhaps.  As I understand it, __packed
> removes padding, but it also forces the compiler to expect the fields
> to be unaligned even if they are actually aligned.

How is that? Andy, mind to point where you get that this
attribute forces compiler to make such assumption?