Re: [Xen-devel] max_grant_frames/max_maptrack_frames

[Jan Beulich <jbeulich@xxxxxxxx>]

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

Jan

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