Re: [Xen-devel] [PATCH 03/11] xen/arm: vpl011: Refactor evtchn_send in Xen to allow sending events from a xen bound channel

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.03.17 at 11:44, <bhupinder.thakur@xxxxxxxxxx> wrote:
> On 6 March 2017 at 13:45, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 05.03.17 at 13:39, <julien.grall@xxxxxxx> wrote:
>>> On 02/21/2017 11:26 AM, Bhupinder Thakur wrote:
>>>> Breakup evtchn_send() to allow sending events for a Xen bound channel. 
>>>> Currently,
>>>> there is a check in evtchn_send() i.e. is_consumer_xen() that if the event 
>>>> channel
>>>> is bound to a xen consumer then event generation is not allowed for that 
>>>> channel.
>>>> This check is to disallow a guest from raising an event via this channel. 
>>>> However,
>>>> it should allow Xen to send a event via this channel as it is required for 
>>>> sending
>>>> vpl011 event to the dom0.
>>>>
>>>> This change introduces a new function raw_evtchn_send() which sends the 
>>>> event
>>>> without this check. The current evtchn_send() calls this function after 
>>>> doing the
>>>> xen consumer check. Xen uses the raw_evtchm_send() version to send the 
>>>> event thus
>>>> bypassing the check.
>>
>> Why would Xen want to send an event it is itself the consumer of?
>> Surely there are better ways to communicate state internally? The
>> more that you say you want the event sent to Dom0...
>>
> As a consumer, Xen receives event from dom0. It also needs to send
> events to dom0 to indicate that there is data in the ring buffer for
> dom0 to read. I am using a xen bound event channel for
> sending/receiving events to/from dom0. I added a new function
> raw_evtchn_send() to allow Xen to send events to dom0 without doing
> the is_xen_consumer check. Note that this check is still there in
> evtchn_send() to disallow guests to raise events on the xen bound
> channel.

I can see why Xen needs to send events; I can't see why Dom0 couldn't
simply make a hypercall instead of sending an event if it needs to signal
something.

Jan


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