[Xen-devel] max_grant_frames/max_maptrack_frames

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

Picking up the discussion from IRC to make it more widely visible...

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. 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.

Thoughts?

  Paul

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