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Subject: Re: [PATCH v7 04/27] x86/fpu/xstate: Introduce XSAVES system states
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, 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>, 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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>
References: <20190606200646.3951-1-yu-cheng.yu@intel.com>
 <20190606200646.3951-5-yu-cheng.yu@intel.com>
 <0a2f8b9b-b96b-06c8-bae0-b78b2ca3b727@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <4effb749-0cdc-6a49-6352-7b2d4aa7d866@intel.com>
Date:   Thu, 6 Jun 2019 15:08:23 -0700
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On 6/6/19 3:04 PM, Andy Lutomirski wrote:
>> But, that seems broken.  If we have supervisor state, we can't 
>> always defer the load until return to userspace, so we'll never?? 
>> have TIF_NEED_FPU_LOAD.  That would certainly be true for 
>> cet_kernel_state.
> 
> Ugh. I was sort of imagining that we would treat supervisor state
 completely separately from user state.  But can you maybe give
examples of exactly what you mean?
> 
>> It seems like we actually need three classes of XSAVE states: 1. 
>> User state
> 
> This is FPU, XMM, etc, right?

Yep.

>> 2. Supervisor state that affects user mode
> 
> User CET?

Yep.

>> 3. Supervisor state that affects kernel mode
> 
> Like supervisor CET?  If we start doing supervisor shadow stack, the 
> context switches will be real fun.  We may need to handle this in 
> asm.

Yeah, that's what I was thinking.

I have the feeling Yu-cheng's patches don't comprehend this since
Sebastian's patches went in after he started working on shadow stacks.

> Where does PKRU fit in?  Maybe we can treat it as #3?

I thought Sebastian added specific PKRU handling to make it always
eager.  It's actually user state that affect kernel mode. :)