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

[David Vrabel <david.vrabel@citrix.com>]

On 05/03/13 17:51, Wei Liu wrote:
> 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.

The user-visible limit option could be toolstack only.  i.e., internally
libxc decides a limit of > 4096 (> 1024 for a 32-bit x86 guest) requires
enabling extended event channels.

David