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Subject: Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET
To:     Andy Lutomirski <luto@kernel.org>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Jann Horn <jannh@google.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        kernel list <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>,
        Borislav Petkov <bp@alien8.de>,
        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>, 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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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Date:   Thu, 3 Sep 2020 09:42:28 -0700
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On 9/3/20 9:32 AM, Andy Lutomirski wrote:
>> Taking the config register out of the init state is illogical, as is
>> writing to SSP while the config register is in its init state.
> What's so special about the INIT state?  It's optimized by XSAVES, but
> it's just a number, right?  So taking the register out of the INIT
> state is kind of like saying "gdb wanted to set xmm0 to (0,0,0,1), but
> it was in the INIT state to begin with", right?

Yeah, that's a good point.  The init state shouldn't be special, as the
hardware is within its right to choose not to use the init optimization
at any time.