Messages in this thread | | | Subject | Re: x2apic_wrmsr_fence vs. Intel manual | From | Jan Kiszka <> | Date | Wed, 4 Mar 2020 19:39:58 +0100 |
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On 04.03.20 19:27, Dave Hansen wrote: > 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? >
Nope, not so far. I'm hunting a race between two guests over Jailhouse where the kick (sent as IPI) seems to come before the data, but changing the fences didn't solve it, unfortunately. Along that, I was reading code and manuals up and down and ran into this inconsistency. That's the story.
Jan
-- Siemens AG, Corporate Technology, CT RDA IOT SES-DE Corporate Competence Center Embedded Linux
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