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Re: [Xen-devel] Questions about device/event channels in Xen.

To: Liang Yang <multisyncfe991@xxxxxxxxxxx>, 'Mark Williamson' <mark.williamson@xxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Questions about device/event channels in Xen.
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Fri, 16 Mar 2007 08:45:31 +0000
Cc: "'Petersson, Mats'" <Mats.Petersson@xxxxxxx>, 'Daniel Stodden' <stodden@xxxxxxxxxx>
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Thread-topic: [Xen-devel] Questions about device/event channels in Xen.
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On 16/3/07 06:02, "Liang Yang" <multisyncfe991@xxxxxxxxxxx> wrote:

> First: it seems Xen uses at least two different types of even "channels".
> First type is for interrupt notification (upper call or uni-directional) and
> the second if for the notification of queued descriptors (bi-directional).
> So is the type of event channel fixed when Xen allocate them or not fixed
> (for the same device), e.g. event channel 2 was a uni-directional type and
> later can be changed to bi-directional type.

An event channel can be allocated/deallocated many times during a domain's
lifetime. The type of an event channel can change across allocations, but is
fixed at allocation time for a particular allocate-to-deallocate period.

> Second: as these events are handled asynchronously, does Xen treat different
> type of event differently?  For example, does Xen always respond to
> interrupt event immediately (unlike queuing more descriptors and then set up
> event)?

Xen doesn't treat event delivery differently depending on type of event
channel. What changes is the reason for kicking the event channel.

> Third: for a PCIe device, I can choose to use MSI or the legacy line-based
> interrupt. Does different type of interrupt handling mechanism affect the
> event channel set-up?

We don't support MSI yet, but the event-channel interface will not change
when MSI is supported. The event channel will still be bound to a 'pirq'.

 -- Keir

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