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Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
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From: Dave Hansen <dave.hansen@intel.com>
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Date: Fri, 9 Nov 2018 09:20:50 -0800
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To: Matthew Wilcox <willy@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>, 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>, Cyrill Gorcunov <gorcunov@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 11/9/18 9:17 AM, Matthew Wilcox wrote:
>> But, later versions of the hardware have instructions that don't have
>> static offsets for the state components (when the XSAVES/XSAVEC
>> instructions are used).  So, for those, the structure embedding isn't
>> used at *all* since some state might not be present.
> But *when present*, this structure is always aligned on an 8-byte
> boundary, right?

There's no guarantee of that.

There is an "aligned" attribute for each XSAVE state component, but I do
not believe it is set for anything yet.