Re: [Xen-devel] max_grant_frames/max_maptrack_frames

[Jürgen Groß <jgross@xxxxxxxx>]

On 08.11.19 12:38, Jan Beulich wrote:
> On 08.11.2019 09:45,  Durrant, Paul  wrote:
> > When per-domain options for maximum grant and maptrack frames came in (in 
> > 4.10?) Xen's behaviour w.r.t. to the global command line values 
> > (gnttab_max_frames and gnttab_max_maptrack_frames respectively) regressed
> >
> > For example, a host running a prior version of Xen with a command line setting 
> > gnttab_max_frames=128 would have all of its domUs running with 128 frames. 
> > However, after update to a newer Xen, they will only get 32 frames (unless the 
> > host is particularly large, in which case they will get 64). Why is this? It's 
> > because neither xl.cfg files, nor xl.conf, will specify values (because the 
> > scenario is an update from an older installation) and so the hardcoded 32/64 
> > default applies. Hence some domUs with large numbers of PV devices start 
> > failing (or at least substantially slow down) and admins start wondering what's 
> > going on.
> >
> > So how best to fix this?
> >
> > For the sake of a quick fix for the regression, and ease of back-porting, I 
> > think it would be best to add a check in domain_create() and create the grant 
> > table with parameters which are the larger of the toolstack configured value 
> > and the corresponding command line value.
> How about people simply setting the value in xl.conf, if indeed in can be
> set there?
>
> > This does, however, go against the recent direction of the toolstack getting 
> > exactly what it asked for. So for the longer term I am wondering whether there 
> > ought to be a way for the toolstack to query the globally configured grant 
> > table limits. A GNTTABOP seems the wrong candidate for this, since GNTTABOPs 
> > are per-domain, so I'm wondering about a new sysctl to return the value of a 
> > named command line parameter.
> Such a series was already posted (and even had some review, so it's
> already at v4, but iirc no update has been provided since May):
> https://lists.xenproject.org/archives/html/xen-devel/2019-05/msg02206.html
My "Hypervisor file system" series includes that functionality:

https://patchew.org/Xen/20191002112004.25793-1-jgross@xxxxxxxx/


Juergen


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