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

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Date:   Fri, 28 Feb 2020 07:53:38 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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 Fri, 2020-02-28 at 13:17 +0100, Borislav Petkov wrote:
> 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.

Yes, saving only supervisor states is optimal, but doing XSAVES with a
partial RFBM changes xcomp_bv.

Yu-cheng