linux-kernel - Re: [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer address finder to support dynamic states

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Date:   Wed, 18 Aug 2021 13:33:27 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     luto@...nel.org, tglx@...utronix.de, mingo@...nel.org,
        x86@...nel.org, len.brown@...el.com, dave.hansen@...el.com,
        thiago.macieira@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer
 address finder to support dynamic states

On Fri, Jul 30, 2021 at 07:59:41AM -0700, Chang S. Bae wrote:
> __raw_xsave_addr() returns the requested component's pointer in an XSTATE
> buffer, by simply looking up the offset table. The offset used to be fixed,
> but, with dynamic user states, it becomes variable.
> 
> get_xstate_size() has a routine to find an offset at runtime. Refactor to
> use it for the address finder.
> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> Reviewed-by: Len Brown <len.brown@...el.com>
> Cc: x86@...nel.org
> Cc: linux-kernel@...r.kernel.org
> ---
> Changes from v5:
> * Updated for future proofed __raw_xsave_addr().
> 
> Changes from v3:
> * Added the function description in the kernel-doc style. (Borislav Petkov)
> * Removed 'no functional change' in the changelog. (Borislav Petkov)
> ---
>  arch/x86/kernel/fpu/xstate.c | 78 ++++++++++++++++++++++++------------
>  1 file changed, 53 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 26f6d5e0f1ed..98ab10e4da3b 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -182,6 +182,38 @@ static bool xfeature_is_supervisor(int xfeature_nr)
>  	return ecx & 1;
>  }
>  
> +/**
> + * get_xstate_comp_offset - Find the feature's offset in the compacted
> + *			    format.
> + * @mask:	This bitmap tells which components reserved in the format.

There's that "reserved" confusion thing. Rewrite pls.

> + * @feature_nr:	The feature number
> + *
> + * Returns:	The offset value
> + */
> +static unsigned int get_xstate_comp_offset(u64 mask, int feature_nr)
> +{
> +	u64 xmask = BIT_ULL(feature_nr + 1) - 1;
> +	unsigned int next_offset, offset = 0;
> +	int i;
> +
> +	if ((xfeatures_mask_all & xmask) == (mask & xmask))
> +		return xstate_comp_offsets[feature_nr];
> +
> +	/*
> +	 * With the given mask, no relevant size is found. Calculate it by
> +	 * summing up each state size.
> +	 */
> +	for (next_offset = FXSAVE_SIZE + XSAVE_HDR_SIZE, i = FIRST_EXTENDED_XFEATURE;
> +	     i <= feature_nr; i++) {
> +		if (!(mask & BIT_ULL(i)))
> +			continue;
> +
> +		offset = xstate_aligns[i] ? ALIGN(next_offset, 64) : next_offset;
> +		next_offset += xstate_sizes[i];

Why is this more complex than it has to be?

IOW, why can't you simply do:

        offset = FXSAVE_SIZE + XSAVE_HDR_SIZE;
        for (i = FIRST_EXTENDED_XFEATURE; i <= feature_nr; i++) {
                if (!(mask & BIT_ULL(i)))
                        continue;

                if (xstate_aligns[i])
                        offset = ALIGN(offset, 64);

                offset += xstate_sizes[i];
        }
        return offset;

like it was before?

-- 
Regards/Gruss,
    Boris.

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