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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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?

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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