Messages in this thread | | | Subject | Re: x2apic_wrmsr_fence vs. Intel manual | From | Dave Hansen <> | Date | Wed, 4 Mar 2020 10:27:08 -0800 |
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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?
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