RE: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is set

[Wu, Feng]

> -----Original Message-----
> From: iommu-bounces@xxxxxxxxxxxxxxxxxxxxxxxxxx
> [mailto:iommu-bounces@xxxxxxxxxxxxxxxxxxxxxxxxxx] On Behalf Of Zhang, Yang Z
> Sent: Thursday, December 18, 2014 11:10 PM
> To: Paolo Bonzini; kvm@xxxxxxxxxxxxxxx
> Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx
> Subject: RE: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is set
> 
> Paolo Bonzini wrote on 2014-12-18:
> >
> >
> > On 18/12/2014 04:14, Wu, Feng wrote:
> >>
> >>
> >> linux-kernel-owner@xxxxxxxxxxxxxxx wrote on
> mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of Paolo:
> >>> x86@xxxxxxxxxx; Gleb Natapov; Paolo Bonzini; dwmw2@xxxxxxxxxxxxx;
> >>> joro-zLv9SwRftAIdnm+yROfE0A@xxxxxxxxxxxxxxxx; Alex Williamson;
> >>> joro-zLv9SwRftAIdnm+Jiang
> >>> Liu
> >>> Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
> >>> linux-kernel-u79uwXL29TY76Z2rM5mHXA@xxxxxxxxxxxxxxxx; KVM list;
> >>> Eric Auger
> >>> Subject: Re: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is
> >>> set
> >>>
> >>>
> >>>
> >>> On 12/12/2014 16:14, Feng Wu wrote:
> >>>> Currently, we don't support urgent interrupt, all interrupts are
> >>>> recognized as non-urgent interrupt, so we cannot send
> >>>> posted-interrupt when 'SN' is set.
> >>>
> >>> Can this happen?  If the vcpu is in guest mode, it cannot have been
> >>> scheduled out, and that's the only case when SN is set.
> >>>
> >>> Paolo
> >>
> >> Currently, the only place where SN is set is vCPU is preempted and
> 
> If the vCPU is preempted, shouldn't the subsequent be ignored? What happens
> if a PI is occurs when vCPU is preempted?

If a vCPU is preempted, the 'SN' bit is set, the subsequent interrupts are
suppressed for posting.

Thanks,
Feng

> 
> >> waiting for the next scheduling in the runqueue. But I am not sure
> >> whether we need to set SN for other purpose in future. Adding SN
> >> checking here is just to follow the Spec. non-urgent interrupts are
> >> suppressed
> > when SN is set.
> >
> > I would change that to a WARN_ON_ONCE then.
> 
> 
> Best regards,
> Yang
> 
> 
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