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:	Fri, 22 Jan 2016 10:03:16 +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 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. If 
it is multicast, then also notifies other vcpus. It is totally handed in 
PI mode and we already have the wakeup vector in /proc/interrupts.

>
>>> For the multicast/broastcast, we cannot set the related IRTE in PI
>>> mode, since we cannot set only one destination in IRTE. If an interrupt
>>> is for multiple destination, how can you use VT-d PI to injection it
>>> to all the destinations?
>>
>> You may still not get my point. Anyway, it doesn't matter. Rollback to
>> legacy mode still is the best choice so far.
>
> I think we can't do much better than we do now.



-- 
best regards
yang