Re: [Xen-devel] grant table size

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

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 20 November 2019 12:09
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] grant table size
> 
> On 20.11.2019 11:49,  Durrant, Paul  wrote:
> >> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> Sent: 20 November 2019 11:06
> >>
> >> Do you have in mind to signal this somehow to guests, or the
> >> expectation is that the guest will have to poll GNTTABOP_query_size
> >> and at some point the size will increase?
> >
> > I don't think the guest need care until its grant table grows to the
> > max. At that point, rather than giving up, the guest would re-query
> > the max value to see if there is now more headroom and then re-size
> > its internal data structures accordingly.
> 
> If we consider dynamic adjustments, what about shrinking of the
> table? This would of course require some form of guest consent,
> but it would be nice if the option would at least be accounted
> for when working out how all of this should behave, even if the
> case may not get handled right now.
> 

Well, perhaps we could have a set_size gnttab op where a guest would be allowed 
to call it with a value less than (or equal to) its current max, so that it can 
voluntarily yield its headroom, but only a privileged guest would be allowed to 
call it with an increased max value?
I'm not sure what mechanism would be best for requesting a guest reduction 
though, I guess probably xenstore... something akin to balloon target pages?

A guest reduction of max is of pretty limited value though AFAICT as only 
in-use frames really use any memory. The (active/shared/status) arrays could, 
of course, be reduced in size but that only gets you a few bytes back.

  Paul
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