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

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Message-ID: <E134A33E-C6B0-4325-A852-E426F305F387@intel.com>
Date:   Wed, 18 Aug 2021 19:47:04 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     "Lutomirski, Andy" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Macieira, Thiago" <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <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 Aug 18, 2021, at 04:33, Borislav Petkov <bp@...en8.de> wrote:
> On Fri, Jul 30, 2021 at 07:59:41AM -0700, Chang S. Bae wrote:
>> 
>> + * @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?

It was refactored to use in the new helper to find feature_nr’s start point.
If the size is added up here, it is not ‘i’'s start point anymore.

Thanks,
Chang