linux-kernel - Re: [PATCH v7 04/27] x86/fpu/xstate: Introduce XSAVES system states

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Date:   Thu, 6 Jun 2019 15:08:23 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v7 04/27] x86/fpu/xstate: Introduce XSAVES system states



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. :)