linux-kernel - Re: [PATCH v5 24/28] x86/fpu/xstate: Use per-task xstate mask for saving xstate in signal frame

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Date:   Mon, 24 May 2021 21:47:53 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Len Brown" <lenb@...nel.org>
Cc:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "Borislav Petkov" <bp@...e.de>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 24/28] x86/fpu/xstate: Use per-task xstate mask for saving xstate in signal frame



On Mon, May 24, 2021, at 11:06 AM, Len Brown wrote:
> On Sun, May 23, 2021 at 11:15 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > If I'm reading this right, it means that tasks that have ever used AMX
> > get one format and tasks that haven't get another one.
> 
> No.  The format of the XSTATE on the signal stack is uncompressed XSAVE
> format for both AMX and non-AMX tasks, both before and after this patch.
> That is because XSAVE gets the format from XCR0.  It gets the fields
> to write from the run-time parameter.
> 
> So the change here allows a non-AMX task to skip writing data (zeros)
> to the AMX region of its XSTATE buffer.

I misread the patch. I still think this patch is useless.

> 
> The subsequent patch adds the further optimization of (manually) checking
> for INIT state for an AMX-task and also skip writing data (zeros) in that case.
> 
> We should have done this optimization for AVX-512, but instead we
> guaranteed writing zeros, which I think is a waste of both transfer time
> and cache footprint.

If no one depends on it, it’s not ABI.