linux-kernel - Re: [PATCH v3] x86/cpu: Use SERIALIZE in sync

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Date:   Thu, 6 Aug 2020 12:57:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Cc:     Tony Luck <tony.luck@...el.com>,
        Cathy Zhang <cathy.zhang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kyung Min Park <kyung.min.park@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-edac@...r.kernel.org
Subject: Re: [PATCH v3] x86/cpu: Use SERIALIZE in sync_core() when available

On 8/6/20 12:25 PM, Ricardo Neri wrote:
>  static inline void sync_core(void)
>  {
>  	/*
> -	 * There are quite a few ways to do this.  IRET-to-self is nice
> +	 * Hardware can do this for us if SERIALIZE is available. Otherwise,
> +	 * there are quite a few ways to do this.  IRET-to-self is nice

This seems to imply that things other than SERIALIZE aren't the hardware
doing this.  All of these methods are equally architecturally
serializing *and* provided by the hardware.

I also don't quite get the point of separating the comments from the
code.  Shouldn't this be:

	/*
	 * The SERIALIZE instruction is the most straightforward way to
	 * do this but it not universally available.
	 */
	if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
		asm volatile(__ASM_SERIALIZE ::: "memory");
		return;
	}

	/*
	 * For all other processors, IRET-to-self is nice ...
	 */
	iret_to_self();