linux-kernel - Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support

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Date:   Mon, 29 Mar 2021 09:31:27 -0400
From:   Len Brown <lenb@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        X86 ML <x86@...nel.org>, "Brown, Len" <len.brown@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to
 control state component support

On Sat, Mar 27, 2021 at 6:20 PM Thomas Gleixner <tglx@...utronix.de> wrote:

> What's the actual downside of issuing TILERELEASE conditionally
> depending on prev->AMX INIT=0? Is it slooooow or what's the real
> problem here?

TILERELEASE is fast, so there should be no down-side to execute it.
Indeed, checking whether you need to execute it or not will probably take
longer than executing TILERELEASE.  My point (perhaps academic)
is that Linux should not have to know about TILERELEASE, or execute it.

Re: running in the kernel with AMX INIT=0

AMX INIT=0 will prevent c6 on that core.  I don't expect to see this
in the syscall path, though if a user wanted to neglect to issue TILERELEASE,
there is nothing forcing them to do so.

It can certainly happen on the interrupt path, but on the interrupt patch
I don't know if we can end up requesting c6 -- perhaps on a forced
task migration?

Re:  frequency credits in the kernel with AMX INIT=0.

It works exactly the same way as AMX INIT=1.
That is to say, the frequency credits don't key off of AMX INIT,
they key off of the actual use of the AMX execution unit, and
the credits free up several orders of magnitude faster
(both for AVX-512 and AMX) on this hardware as in previous generations.

As a result, if we interrupt an AMX program, and run for an extended
period of time in the kernel without XRESTOR to clear out his AMX INIT=0 state,
that will not have any impact on the frequency we run inside the kernel any more
than if he had AMX INIT=1 state.

thanks,
Len Brown, Intel Open Source Technology Center