Re: [Xen-devel] grant table size

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

> -----Original Message-----
> From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Sent: 20 November 2019 11:06
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] grant table size
> 
> On Wed, Nov 20, 2019 at 09:43:59AM +0000, Durrant, Paul wrote:
> > I've dealt with a few problems over the years where the root cause was a
> guest running out of grant table and so I'm wondering whether it would be
> a good idea to allow a toolstack to increase the table size of a running
> guest, e.g. when plugging in a new PV interface.
> I would rather have a new xl command that does the grant table
> increase (ie: xl set-max-grant-frames) instead of doing it when
> plugging new interfaces.
>

That would be ok too... Just thought it might be nicer if it were automatic but 
it would indeed be complete guess-work in libxl to come up with a per-interface 
grant table quota.
 
> > It would appear that current Linux guests would not be able to make use
> of this as it stands (but that could be fixed), but as far as I can tell a
> pvops kernel would not misbehave if the maximum table size were to
> increase. Similarly Windows PV drivers would need modification to make use
> of a dynamic maximum table size but would not misbehave as is.
> > Does anyone have any objection to the idea?
> 
> 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.

  Paul



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