linux-kernel - Re: [PATCH v9 08/26] x86/fpu/xstate: Introduce helpers to manage the XSTATE buffer dynamically

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Message-ID: <YS0QRWjuqh9ZzIGE@zn.tnic>
Date:   Mon, 30 Aug 2021 19:07:17 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
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 08/26] x86/fpu/xstate: Introduce helpers to manage the
 XSTATE buffer dynamically

On Wed, Aug 18, 2021 at 07:46:55PM +0000, Bae, Chang Seok wrote:
> No, this non-compacted thing is not for optimization. SDM is not quite clear
> about the logic behind the non-compacted format -- some state’s offset does
> not always match with the 'size + offset' of the previous one, even without
> 64B-alignment. So, the loop is only for the compacted format, not the
> non-compacted one. 
> 
> 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.

Let's see, we're still talking about this thing, right:

        nr = fls64(mask) - 1;

        if (!boot_cpu_has(X86_FEATURE_XSAVES))
                return xstate_offsets[nr] + xstate_sizes[nr];

?

That @mask is "which components reserved in the buffer."

Which buffer? The mask being passed is independent from whatever buffer.

So you need to do a lot more explaining here before this goes anywhere.

-- 
Regards/Gruss,
    Boris.

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