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Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack
To:     Andy Lutomirski <luto@amacapital.net>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     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>
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Message-ID: <043a17ef-dc9f-56d2-5fba-1a58b7b0fd4d@intel.com>
Date:   Thu, 8 Nov 2018 13:48:54 -0800
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On 11/8/18 1:22 PM, Andy Lutomirski wrote:
>> +struct cet_kernel_state {
>> +       u64 kernel_ssp; /* kernel shadow stack */
>> +       u64 pl1_ssp;    /* ring-1 shadow stack */
>> +       u64 pl2_ssp;    /* ring-2 shadow stack */
>> +} __packed;
>> +
> Why are these __packed?  It seems like it'll generate bad code for no
> obvious purpose.

It's a hardware-defined in-memory structure.  Granted, we'd need a
really wonky compiler to make that anything *other* than a nicely-packed
24-byte structure, but the __packed makes it explicit.

It is probably a really useful long-term thing to stop using __packed
and start using "__hw_defined" or something that #defines down to __packed.