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SubjectRE: [PATCH] KVM: x86: Sync the pending Posted-Interrupts
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> >>> However, you should at least change the comment in vcpu_enter_guest
> >>> to mention "before reading PIR" instead of "before reading PIR.ON".
> >>
> >> Will do that. I think the "checking PIR.ON" should be PID.ON. I will fix it.
> >>
> > Hi Paolo,
> > I reconsidered the comment in vcpu_enter_guest() and think it may don't need to change. The original comment as below:
> > * 2) For APICv, we should set ->mode before checking PIR.ON. This
> > * pairs with the memory barrier implicit in pi_test_and_set_on
> > * (see vmx_deliver_posted_interrupt).
> >
> > I think "before checking PIR.ON" is mean "before checking PID.PIR and PID.ON".
>
> I can say it only means PID.ON because I wrote the comment. :)
>
> The idea is that checking ON is enough: KVM assumes that PID.PIR is only set if PID.ON is set, because it follows the definition of ON in table
> 29-1 of the SDM: "If this bit is set, there is a notification outstanding for one or more posted interrupts in bits 255:0".
>
> VT-D breaks this assumption whenever SN=1 ("hardware does not generate notification event nor modify the ON field"), resulting in
> nonzero PID.PIR but PID.ON=0. I'm sure there was a reason for that, but it does result in inconsistency between the PID definitions in the
> SDM and the VT-D specification. The right fix is definitely to reconcile this difference and test the bitmap after SN is cleared (with
> smp_mb__after_atomic after clearing SN), and set ON=1 if the bitmap is not clear.
>
Hi Paolo
Thanks for your elaboration, very clear to me. I will fix it in next version.

Thanks,
Luwei Kang



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