Re: [Xen-devel] max_grant_frames/max_maptrack_frames

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

> -----Original Message-----
> From: Jürgen Groß <jgross@xxxxxxxx>
> Sent: 08 November 2019 12:14
> To: Jan Beulich <jbeulich@xxxxxxxx>; Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] max_grant_frames/max_maptrack_frames
> 
> 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/
> 

Oh, even better :-)

  Paul

> 
> Juergen

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