Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/05/21 19:53, Marcelo Tosatti wrote:
> On Mon, May 24, 2021 at 05:55:18PM +0200, Paolo Bonzini wrote:
>> On 10/05/21 19:26, Marcelo Tosatti wrote:
>>> +void vmx_pi_start_assignment(struct kvm *kvm)
>>> +{
>>> +	struct kvm_vcpu *vcpu;
>>> +	int i;
>>> +
>>> +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
>>> +		return;
>>> +
>>> +	/*
>>> +	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
>>> +	 * go back through vcpu_block().
>>> +	 */
>>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>>> +		if (!kvm_vcpu_apicv_active(vcpu))
>>> +			continue;
>>> +
>>> +		kvm_vcpu_wake_up(vcpu);
>>
>> Would you still need the check_block callback, if you also added a
>> kvm_make_request(KVM_REQ_EVENT)?
>>
>> In fact, since this is entirely not a hot path, can you just do
>> kvm_make_all_cpus_request(kvm, KVM_REQ_EVENT) instead of this loop?
>>
>> Thanks,
>>
>> Paolo
> 
> Hi Paolo,
> 
> Don't think so:
> 
> static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> {
>          int ret = -EINTR;
>          int idx = srcu_read_lock(&vcpu->kvm->srcu);
> 
>          if (kvm_arch_vcpu_runnable(vcpu)) {
>                  kvm_make_request(KVM_REQ_UNHALT, vcpu);  <---- don't want KVM_REQ_UNHALT

UNHALT is incorrect indeed, but requests don't have to unhalt the vCPU.

This case is somewhat similar to signal_pending(), where the next 
KVM_RUN ioctl resumes the halt.  It's also similar to 
KVM_REQ_PENDING_TIMER.  So you can:

- rename KVM_REQ_PENDING_TIMER to KVM_REQ_UNBLOCK except in 
arch/powerpc, where instead you add KVM_REQ_PENDING_TIMER to 
arch/powerpc/include/asm/kvm_host.h

- here, you add

	if (kvm_check_request(KVM_REQ_UNBLOCK, vcpu))
		goto out;

- then vmx_pi_start_assignment only needs to

	if (!irq_remapping_cap(IRQ_POSTING_CAP))
		return;
	kvm_make_all_cpus_request(kvm, KVM_REQ_UNBLOCK);

kvm_arch_vcpu_runnable() would still return false, so the mp_state would 
not change.

Paolo