Re: [Xen-devel] [RFC PATCH V4 18/18] libxl: add evtchn_extended_allowed flag

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Mar 05, 2013 at 05:38:54PM +0000, Ian Jackson wrote:
> Wei Liu writes ("Re: [RFC PATCH V4 18/18] libxl: add evtchn_extended_allowed 
> flag"):
> > On Tue, 2013-03-05 at 13:48 +0000, Ian Jackson wrote:
> > > This fails to explain why one might want to disable this.  The only
> > > reason that comes to my mind is in case it has a security
> > > vulnerability, an admin who wasn't currently using it could disable
> > > it.
> > > 
> > > Are there other reasons ?
> > 
> > It is not for security reason.
> > 
> > The main concern is that a) extended event channel might use too much
> > global mapping space in Xen; b) in 3-level ABI's case, normal DomU will
> > never consume so many event channels.
> 
> This is rather opaque from the documentation as proposed.  Perhaps a
> limit on the total number of event channels for a domain would make
> more sense ?

I will improve the documentation. But putting a limit on total number of
event channels for a domain for now is not what I expect, because a)
having limit on 2/3-level event channels brings no significant
improvement, b) the infrastructure to notify a guest about its limit
doesn't exists.

> 
> > > This is an odd phrasing given that there is (currently in existence)
> > > only one extended event channel ABI.  And in the future when we
> > > introduce more we probably want to be able to disable them
> > > individually ?
> > 
> > This option in fact was introduced as 3-level event channel ABI only,
> > the name was evtchn_l3 or something at first. But Jan later suggested
> > naming it something more generic.
> 
> Well, to an extent we're trying to predict what we might want to
> enable/disable in the future.
> 

It is hard to predict, as 3-level ABI is the only extended ABI exists at the
moment...

> > If we are sure that we want to enable extended event channel for all
> > later ABIs, this option can be restrict to 3-level ABI.
> 
> Maybe we want to enable/disable them separately.
> 

Then let's restrict it to 3-level ABI, say, name it
evtchn_extended_l3_allowed.


Wei.

> Ian.

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