Re: [Xen-devel] [PATCH v1] libxl: remove limit for default number of event channels

[Julien Grall <julien@xxxxxxx>]

Hi,

On 27/03/2020 14:37, Olaf Hering wrote:
> On Fri, Mar 27, Ian Jackson wrote:
>
> > This seems likely to be right, but: what is the default in Xen ?  Is
> > it sufficiently tight to stop a guest using too many resources ?
> The value of d->max_evtchns will be either 4k or 128k.
> AFAICS no extra resources are allocated with the changed value.
Event channels are allocated using buckets. Everytime you will a bucket, 
a new bucket will be allocated.
By increasing the limit, you effectively increase the number of buckets 
than can be allocated.
So while I agree that the default allocation will be the same, you 
effectively allow the guest to use more resource in Xen. Therefore I 
don't think this is acceptable to lift the limits for all the guests.
Cheers,

--
Julien Grall