linux-kernel - Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if the interrupt is not single-destination

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Date:	Mon, 25 Jan 2016 09:49:53 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	"rkrcmar@...hat.com" <rkrcmar@...hat.com>
Cc:	"Wu, Feng" <feng.wu@...el.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 1/4] KVM: Recover IRTE to remapped mode if the
 interrupt is not single-destination

On 2016/1/22 21:31, rkrcmar@...hat.com wrote:
> 2016-01-22 10:03+0800, Yang Zhang:
>> On 2016/1/22 0:35, rkrcmar@...hat.com wrote:
>>> 2016-01-21 13:44+0800, Yang Zhang:
>>>> On 2016/1/21 13:41, Wu, Feng wrote:
>>>>>> From: Yang Zhang [mailto:yang.zhang.wz@...il.com]
>>>>>> We may have different understanding on PI mode. My understanding is if
>>>>>> we set the IRTE to PI format, than the subsequent interrupt will be
>>>>>> handled in PI mode. multi-cast and broadcast interrupts cannot be
>>>>>> injected to guest directly but it doesn't mean cannot be handled in PI
>>>>>> mode. As i said, we can handle it in wake up vector or via other
>>>>>> approach.But it is much complexity.
>>>
>>> KVM has to intercept the interrupt, so we'd need to trigger a deferred
>>> work from the notification handler to send the multicast.
>>> Reusing existing PI vectors would mean slowing them down, so we should
>>> define a new PI notification vector just for this purpose, which would
>>> be confusing in /proc/interrupts anyway.
>>> On top of that, we'd need to define new PIRR array(s) and create unique
>>> PID for every IRTE, to avoid parsing those PIRR arrays as the vector is
>>> stored in IRTE ... it's going a bit too far, I guess.
>>
>> Not so complicated. We can reuse the wake up vector and check whether the
>> interrupt is multicast when one of destination vcpu handles it.
>
> I'm not sure what you mean now ... I guess it is:
> - Deliver the interrupt to a guest VCPU and relay the multicast to other
>    VCPUs.  No, it's strictly worse than intercepting it in the host.

It is still handled in host context not guest context. The wakeup event 
cannot be consumed like posted event. So it relies on hypervisor to 
inject the interrupt to guest. We can add the check at this point.


>
> - Modify host's wakeup vector handler to send the multicast.
>    It's so complicated, because all information you start with in the
>    host is a vector number.  You start with no idea what the multicast
>    interrupt should be.
>
>    We could add per-multicast PID to the list of parsed PIDs in
>    wakeup_handler and use PID->multicast interrupt mapping to tell which
>    interrupt we should send, but that seems worse than just delivering a
>    non-remapped interrupt.
>
>    Also, if wakeup vector were used for wakeup and multicast, we'd be
>    uselessly doing work, because we can't tell which reason triggered the
>    interrupt before finishing one part -- using separate vectors for that
>    would be a bit nicer.
>


-- 
best regards
yang