linux-kernel - Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore

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Message-ID: <20200228121724.GA25261@zn.tnic>
Date:   Fri, 28 Feb 2020 13:17:24 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
 __fpu__restore_sig()

On Thu, Feb 27, 2020 at 02:52:12PM -0800, Yu-cheng Yu wrote:
> > So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
> > this?
> > 
> > Why don't you simply do:
> > 
> > 		set_thread_flag(TIF_NEED_FPU_LOAD);
> > 		fpregs_lock();
> > 		if (xfeatures_mask_supervisor())
> > 			copy_xregs_to_kernel(&fpu->state.xsave);
> > 		fpregs_unlock();
> 
> If TIF_NEED_FPU_LOAD is set, then xstates are already in the xsave buffer. 
> We can skip saving them again.

Ok, then pls use test_and_set_thread_flag().

Also, in talking to Sebastian about this on IRC, he raised a valid
concern: if we are going to save supervisor states here, then
copy_xregs_to_kernel() should better save *only* supervisor states
because we're not interested in the user states - they're going to be
overwritten with the states from the stack.

So copy_xregs_to_kernel() needs to learn about a second parameter called
@mask like copy_kernel_to_xregs().

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette