Re: [PATCH v2 2/3] KVM: VMX: Do not change PID.NDST when loading a blocked vCPU

[Joao Martins]

On 11/19/19 11:36 AM, Wanpeng Li wrote:
> On Tue, 12 Nov 2019 at 01:23, Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>>
>> While vCPU is blocked (in kvm_vcpu_block()), it may be preempted which
>> will cause vmx_vcpu_pi_put() to set PID.SN.  If later the vCPU will be
> 
> How can this happen? See the prepare_to_swait_exlusive() in
> kvm_vcpu_block(), the task will be set in TASK_INTERRUPTIBLE state,
> kvm_sched_out will set vcpu->preempted to true iff current->state ==
> TASK_RUNNING.

You're right.

But preemption (thus setting PID.SN) can still happen before vcpu_block(), or
otherwise scheduled out if we are already in vcpu_block() (with task in
TASK_INTERRUPTIBLE). The right term we should have written in that sentence
above would have been 'scheduled out' and drop the vcpu_block mention and it
would encompass both cases. A better sentence would perhaps be:

"While vCPU is blocked (or about to block) it may be scheduled out which will
cause vmx_vcpu_pi_put() to be called."

But setting or not preempted/PID.SN doesn't change the rest and was mentioned
for completeness.

	Joao