Re: x2apic_wrmsr_fence vs. Intel manual

[Jan Kiszka]

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

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