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Date: Wed, 20 Feb 2019 17:21:28 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: [MODERATED] Re: [patch V2 09/10] MDS basics+ 9
Message-ID: <20190220162128.GW32477@hirez.programming.kicks-ass.net>
References: <20190220150753.665964899@linutronix.de>
 <20190220151400.787843957@linutronix.de>
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To: speck@linutronix.de
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On Wed, Feb 20, 2019 at 04:08:02PM +0100, speck for Thomas Gleixner wrote:
>  static inline void mds_user_clear_cpu_buffers(void)
>  {
> +	if (static_branch_likely(&mds_user_clear_cond)) {

Are we sure we want _likely() ? That puts the body in-line.

> +		if (__this_cpu_read(mds_cond_clear)) {
> +			__this_cpu_write(mds_cond_clear, 0);
> +			mds_clear_cpu_buffers();
> +		}
		return;

_might_ generate better code; by making it explitit we'll never have
both branches enabled at the same time.

> +	}
>  	if (static_branch_likely(&mds_user_clear_always))
>  		mds_clear_cpu_buffers();
>  }

And in general; yuck! Should I once again look at tri-state jump_labels
or static_switch? Last time I tried I got stuck on the C part, writing
different targets is trivial.

Ideally we'd end up with something like:

1:	jmp 3f
	test $1, %gs:mds_cond_clear
	jz 3f
	mov $0, %gs:mds_cond_clear
2:	verw ds
3:

And change the jmp at 1 between: JMP 3f, JMP 2f and NOP for
mds={off,full,cond} resp.