Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> Sent: Tuesday, September 22, 2015 10:28 PM
> To: Wu, Feng; Dario Faggioli
> Cc: xen-devel@xxxxxxxxxxxxx; Tian, Kevin; Keir Fraser; George Dunlap; Andrew
> Cooper; Jan Beulich
> Subject: Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core 
> logic
> handling
> 
> On 09/22/2015 02:25 PM, Wu, Feng wrote:
> >>> But if we want to avoid spurious PI interrupts when running idle, then
> >>> yes, we need *some* kind of a hook on the lazy context switch path.
> >>>
> >>> /me does some more thinking...
> >>
> >> To be honest, since we'll be get spurious PI interrupts in the
> >> hypervisor all the time anyway, I'm inclined at the moment not to worry
> >> to much about this case.
> >
> > Why do you say "we'll be get spurious PI interrupts in the  hypervisor all 
> > the
> time"?
> >
> > And could you please share what is your concern to handle this case to avoid
> > such spurious PI interrupts? Thanks!
> 
> So please correct me if I'm wrong in my understanding:
> 
> * When a vcpu is in runstate "running", with PI enabled, you have the PI
> vector set to "posted_interrupt_vector", SN=0.
> 
> * When in this state in non-root mode, PI interrupts result in an
> interrupt being delivered directly to the guest.
> 
> * When in this state in root mode, PI interrupts result in a
> posted_interrupt_vector interrupt being delivered to Xen.
> 
> Is that the case?

Exactly, it is how PI works today.

Thanks,
Feng

> 
> So basically, if the PI happens to come in when the guest is making a
> hypercall, or the guest is doing any other number of things that involve
> the hypervisor, then Xen will get a "spurious" PI interrupt -- spurious
> because there's nothing Xen actually needs to do about it; the guest
> interrupt will be delivered the next time we do a VMENTER.
> 
> So spurious PI interrupts are already going to happen from time to time
> -- they're not really that big of a deal.  Having them happen when a VM
> is running a tasklet or idle waiting for qemu isn't such a big deal either.



> 
>  -George

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