linux-kernel - Re: [patch V2 01/52] x86/fpu: Make init_fpstate correct with optimized XSAVE

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Date:   Mon, 14 Jun 2021 21:15:27 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch V2 01/52] x86/fpu: Make init_fpstate correct with
 optimized XSAVE

On Mon, Jun 14, 2021 at 05:44:09PM +0200, Thomas Gleixner wrote:
> @@ -466,10 +489,20 @@ static void __init setup_init_fpu_buf(vo
>  	copy_kernel_to_xregs_booting(&init_fpstate.xsave);
>  
>  	/*
> -	 * Dump the init state again. This is to identify the init state
> -	 * of any feature which is not represented by all zero's.
> +	 * All components are now in init state. Read the state back so
> +	 * that init_fpstate contains all non-zero init state. This is only
> +	 * working with XSAVE,

"This only works with XSAVE, ... "


> but not with XSAVEOPT and XSAVES because
> +	 * those use the init optimization which skips writing data for
> +	 * components in init state.

<--- Add a newline in the comment here so that it is not as dense.

> So XSAVE could be used, but that would
> +	 * require to reshuffle the data when XSAVES is available because
> +	 * XSAVES uses xstate compaction. But doing so is a pointless
> +	 * exercise because most components have an all zeros init state
> +	 * except for the legacy ones (FP and SSE). Those can be saved with
> +	 * FXSAVE into the legacy area. Adding new features requires to
> +	 * ensure that init state is all zeroes or if not to add the
> +	 * necessary handling here.
>  	 */
> -	copy_xregs_to_kernel_booting(&init_fpstate.xsave);
> +	fxsave_to_kernel(&init_fpstate.fxsave);

With those fixed:

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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