Re: [PATCH 2/2] x86, apicv: Add Posted Interrupt supporting

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 13, 2012 at 03:29:40PM +0800, Yang Zhang wrote:
> From: Yang Zhang <yang.z.zhang@Intel.com>
> 
> Posted Interrupt allows APIC interrupts to inject into guest directly
> without any vmexit.
> 
> - When delivering a interrupt to guest, if target vcpu is running,
>   update Posted-interrupt requests bitmap and send a notification event
>   to the vcpu. Then the vcpu will handle this interrupt automatically,
>   without any software involvemnt.
> 
> - If target vcpu is not running or there already a notification event
>   pending in the vcpu, do nothing. The interrupt will be handled by
>   next vm entry.
> 
> Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
> ---

<snip>

> +static void pi_handler(void)
> +{
> +	;
> +}
> +
> +static int vmx_has_posted_interrupt(struct kvm_vcpu *vcpu)
> +{
> +	return irqchip_in_kernel(vcpu->kvm) && enable_apicv_pi;
> +}
> +
> +static int vmx_send_nv(struct kvm_vcpu *vcpu,
> +		int vector)
> +{
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +
> +	pi_set_pir(vector, vmx->pi);

Section 29.6 "Posted interrupt processing":

"No other agent can read or write a PIR bit (or groups of bits) between
the time it is read (to determine what to OR into VIRR) and when it is
cleared".

> +	if (!pi_test_and_set_on(vmx->pi) && (vcpu->mode == IN_GUEST_MODE)) {
> +		apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), POSTED_INTR_VECTOR);
> +		return 1;
> +	}
> +	return 0;

What is the purpose of outstanding-notification bit? At first, its use as a
"lock" for PIR posted-interrupt bits is limited because its cleared
on step 3. before PIR is cleared. If it were cleared after step 5. then
software could

	if (!pi_test_and_set_on(vmx->pi)) {
		pi_set_pir(vector, vmx->pi);
		apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), POSTED_INTR_VECTOR);
	}

Does this mean software has to read PIR _and_ outstanding notification
bit to know when its possible to set bits in PIR + send IPI?

Or is it really cleared after step 5?