linux-kernel - Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

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Date:   Thu, 25 Mar 2021 15:26:08 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Len Brown <lenb@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        X86 ML <x86@...nel.org>, "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the
 first use of dynamic user state



>>> For AMX, we must still reserve the space, but we are not going to write zeros
>>> for clean state.  We so this in software by checking XINUSE=0, and clearing
>>> the xstate_bf for the XSAVE.  As a result, for XINUSE=0, we can skip
>>> writing the zeros, even though we can't compress the space.
>> So my understanding is that clearing xstate_bv will not help prevent saving
>> zeros, but only not masking EDX:EAX, since the following logic. Not sure if
>> this is just what you mean. :)
> FWIW, PATCH21 [1] uses the instruction mask to skip writing zeros on sigframe.
> Then, XSAVE will clear the xstate_bv for the xtile data state bit.
>
> [1] https://lore.kernel.org/lkml/20210221185637.19281-22-chang.seok.bae@intel.com/
Yes, no mask in EDX:EAX works in such case. Thanks for pointing out the 
patch.

BRs,
Jing
>
> Thanks,
> Chang