Re: [Xen-devel] [PATCH v14 1/2] vmx: VT-d posted-interrupt core logic handling

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Wu, Feng
> Sent: Monday, February 29, 2016 11:00 AM
> 
> This is the core logic handling for VT-d posted-interrupts. Basically it
> deals with how and when to update posted-interrupts during the following
> scenarios:
> - vCPU is preempted
> - vCPU is slept
> - vCPU is blocked
> 
> When vCPU is preempted/slept, we update the posted-interrupts during
> scheduling by introducing two new architecutral scheduler hooks:
> vmx_pi_switch_from() and vmx_pi_switch_to(). When vCPU is blocked, we
> introduce a new architectural hook: arch_vcpu_block() to update
> posted-interrupts descriptor.
> 
> Besides that, before VM-entry, we will make sure the 'NV' filed is set
> to 'posted_intr_vector' and the vCPU is not in any blocking lists, which
> is needed when vCPU is running in non-root mode. The reason we do this check
> is because we change the posted-interrupts descriptor in vcpu_block(),
> however, we don't change it back in vcpu_unblock() or when vcpu_block()
> directly returns due to event delivery (in fact, we don't need to do it
> in the two places, that is why we do it before VM-Entry).
> 
> When we handle the lazy context switch for the following two scenarios:
> - Preempted by a tasklet, which uses in an idle context.
> - the prev vcpu is in offline and no new available vcpus in run queue.
> We don't change the 'SN' bit in posted-interrupt descriptor, this
> may incur spurious PI notification events, but since PI notification
> event is only sent when 'ON' is clear, and once the PI notificatoin
> is sent, ON is set by hardware, hence no more notification events
> before 'ON' is clear. Besides that, spurious PI notification events are
> going to happen from time to time in Xen hypervisor, such as, when
> guests trap to Xen and PI notification event happens, there is
> nothing Xen actually needs to do about it, the interrupts will be
> delivered to guest atht the next time we do a VMENTRY.
> 
> CC: Keir Fraser <keir@xxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> CC: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> Suggested-by: Yang Zhang <yang.z.zhang@xxxxxxxxx>
> Suggested-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> Suggested-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>

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