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Subject: Re: [PATCH v7 04/27] x86/fpu/xstate: Introduce XSAVES system states
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <0a2f8b9b-b96b-06c8-bae0-b78b2ca3b727@intel.com>
Date:   Thu, 6 Jun 2019 15:04:41 -0700
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        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>,
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        Kees Cook <keescook@chromium.org>,
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        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>
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To:     Dave Hansen <dave.hansen@intel.com>
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On Jun 6, 2019, at 2:18 PM, Dave Hansen <dave.hansen@intel.com> wrote:

>> +/*
>> + * Helpers for changing XSAVES system states.
>> + */
>> +static inline void modify_fpu_regs_begin(void)
>> +{
>> +    fpregs_lock();
>> +    if (test_thread_flag(TIF_NEED_FPU_LOAD))
>> +        __fpregs_load_activate();
>> +}
>> +
>> +static inline void modify_fpu_regs_end(void)
>> +{
>> +    fpregs_unlock();
>> +}
>=20
> These are massively under-commented and under-changelogged.  This looks
> like it's intended to ensure that we have supervisor FPU state for this
> task loaded before we go and run the MSRs that might be modifying it.
>=20
> 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 wha=
t you mean?

>=20
> It seems like we actually need three classes of XSAVE states:
> 1. User state

This is FPU, XMM, etc, right?

> 2. Supervisor state that affects user mode

User CET?


> 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.

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

=E2=80=94Andy