linux-kernel - Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate

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Message-ID: <YK+I8YEEnhWjuVAd@hirez.programming.kicks-ass.net>
Date:   Thu, 27 May 2021 13:56:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Len Brown <lenb@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...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>,
        "Chen, Tim C" <tim.c.chen@...el.com>
Subject: Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when
 appropriate

On Mon, May 24, 2021 at 02:24:54PM -0400, Len Brown wrote:
> On Mon, May 24, 2021 at 1:39 PM Dave Hansen <dave.hansen@...el.com> wrote:
> 
> > >> might be considered nasty.
> 
> > > I'm not excited about burdening the generic idle path with a CPU
> > > feature specific check that would need to be checked on every idle
> > > entry.
> >
> > Me neither.
> >
> > But, the check itself should be cheap.  A cpu_feature_enabled(AMX) check
> > will eliminate even the cost of a branch on systems without AMX.  You
> > could probably even get fancy and also use a static branch that doesn't
> > get enabled until the first AMX user shows up.
> 
> It isn't just the hardware run-time cost.
> It is the source code complexity.
> That code is 100% generic.

Why can't this live in intel_idle.c ? We had to pull out
CPUIDLE_FLAG_TLB_FLUSH because leave_mm() requires RCU, but afaict both
TILERELEASE and XRESTOR do not and could be added to intel_idle_tile(),
which can be used in XXX_cstates[] for the relevant chips instead of
intel_idle() for C6 and up.

intel_idle_tile(args)
{
	bool has_tile = // something XINUSE

	if (has_tile)
		TILERELEASE

	intel_idle(args...);

	if (has_tile)
		// something XRESTOR
}

Hmm?