linux-kernel - Re: x2apic_wrmsr_fence vs. Intel manual

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Date:   Wed, 4 Mar 2020 10:27:08 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jan Kiszka <jan.kiszka@...mens.com>, x86 <x86@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x2apic_wrmsr_fence vs. Intel manual

On 3/2/20 8:11 AM, Jan Kiszka wrote:
> The former dates back to ce4e240c279a, but that commit does not mention 
> why lfence is not needed. Did the manual read differently back then? Or 
> why are we safe? To my reading of lfence, it also has a certain 
> instruction serializing effect that mfence does not have.

I asked around Intel about this.

The old "SFENCE, or MFENCE" recommendation was deemed insufficient
because it has no impact on the ordering of WRMSR since it is not a
"load or store instruction".  LFENCE's instruction-ordering semantic is
needed because it ensures later ordering of all instructions, not just
loads and stores.

Jan, do you think you're seeing a bug resulting from WRMSR ordering?