linux-kernel - Re: [PATCH v2 5/5] KVM: nVMX: Enable nested posted interrupt processing.

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Date:	Wed, 21 Jan 2015 18:35:12 +0800
From:	Wincy Van <fanwenyi0529@...il.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Jan Kiszka <jan.kiszka@....de>
Subject: Re: [PATCH v2 5/5] KVM: nVMX: Enable nested posted interrupt processing.

On Wed, Jan 21, 2015 at 4:49 PM, Zhang, Yang Z <yang.z.zhang@...el.com> wrote:
>>>> +       if (vector == vmcs12->posted_intr_nv && +
>>>> nested_cpu_has_posted_intr(vmcs12)) { +               if (vcpu->mode
>>>> == IN_GUEST_MODE) + apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), +
>>>>                           POSTED_INTR_VECTOR); +               else {
>>>> +                       r = -1; +                       goto out; +
>>>>            } + +               /* +                * if posted intr is
>>>> done by hardware, the +                * corresponding eoi was sent to
>>>> L0. Thus +                * we should send eoi to L1 manually. +
>>>>          */ +               kvm_apic_set_eoi_accelerated(vcpu, +
>>>>                 vmcs12->posted_intr_nv);
>>>
>>> Why this is necessary? As your comments mentioned, it is done by
>>> hardware not L1, why L1 should aware of it?
>>>
>>
>> According to SDM 29.6, if the processor recognizes a posted interrupt,
>> it will send an EOI to LAPIC.
>> If the posted intr is done by hardware, the processor will send eoi to
>> hardware LAPIC, not L1's, just like the none-nested case(the physical
>> interrupt is dismissed). So we should take care of the L1's LAPIC and send an eoi to it.
>
> No. You are not emulating the PI feature. You just reuse the hardware's capability. So you don't need to let L1 know it.
>

Agreed, I had thought we have already set L1's IRR before this, I was wrong.

BTW, I was trying to complete the nested posted intr manually if the
dest vcpu is in_guest_mode but not IN_GUEST_MODE, but I found that
it is difficult to set RVI of the destination vcpu timely, because we
should keep the RVI, PIR and ON in sync : (

I think it is better to do a nested vmexit in the case above, rather
than emulate it, because that case is much less than the hardware
case.


Thanks,

Wincy.
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