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RE: [Xen-devel] Question about evtchn_callback reentry

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Question about evtchn_callback reentry
From: "Xu, Anthony" <anthony.xu@xxxxxxxxx>
Date: Mon, 18 Dec 2006 17:53:49 +0800
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, xen-ia64-devel <xen-ia64-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] Question about evtchn_callback reentry
Keir Fraser write on 2006年12月18日 17:32:
> On 18/12/06 5:28 am, "Xu, Anthony" <anthony.xu@xxxxxxxxx> wrote:

Thanks for your quick response

>> If there is no special handling, low privileged interrupt may
>> "interrupt" high privileged interrupt.
> There's no concept of privilege among event channels. Since Xen simply
> allocates ExtInts to vectors in a round-robin manner, there is no
> meaningful priority comparsion to be made between vectors anyway.

Yes there's no concept of privilege among event channel,

But all device interrupts have privilege and  device interrupts are delivered 
through event channel.

That means ide interrupt may interrupt time interrup in dom0.
It maybe not reasonable.

>> BTW,
>> I do see evtchn_callback reentry happen on IPF dom0, and that incur
>> about 5% performance degradation
>> with KB on VTI-domain compared to non_reentry.
> KB == Keyboard?!
Sorry for confusion,
KB means Kernal Build.

> Reentry is allowed -- it shouldn't happen often unless the ISRs are
> really long (bad) or we are hammering the interrupt line for some
> reason (bad). 

If evtchn_callback reentry is allowed.
There are 1024 event channels.
In theory, dom0 kernal stack may be overflowed.
Is there any mechanism to prevent this?

Since event channels have no concept of privilege.
Why we allow evtchn_callback reentered ?

I think evtchn_callback can handle all events in batch.
This is good for performance.

-- Anthony

>  -- Keir

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