Re: [PATCH] xen/grants: repurpose command line max options

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Mar 14, 2023 at 11:04:21AM +0100, Jan Beulich wrote:
> On 14.03.2023 10:22, Roger Pau Monné wrote:
> > On Mon, Mar 13, 2023 at 05:55:09PM +0100, Jan Beulich wrote:
> >> On 13.03.2023 13:16, Roger Pau Monne wrote:
> >>> --- a/docs/misc/xen-command-line.pandoc
> >>> +++ b/docs/misc/xen-command-line.pandoc
> >>> @@ -1232,9 +1232,8 @@ The usage of gnttab v2 is not security supported on 
> >>> ARM platforms.
> >>>  
> >>>  > Can be modified at runtime
> >>>  
> >>> -Specify the maximum number of frames which any domain may use as part
> >>> -of its grant table. This value is an upper boundary of the per-domain
> >>> -value settable via Xen tools.
> >>> +Specify the default maximum number of frames which any domain may use as 
> >>> part
> >>> +of its grant table unless a different value is specified at domain 
> >>> creation.
> >>>  
> >>>  Dom0 is using this value for sizing its grant table.
> >>
> >> dom0less DomU-s do as well, at the very least, also ...
> >>
> >>> @@ -1245,9 +1244,10 @@ Dom0 is using this value for sizing its grant 
> >>> table.
> >>>  
> >>>  > Can be modified at runtime
> >>>  
> >>> -Specify the maximum number of frames to use as part of a domains
> >>> -maptrack array. This value is an upper boundary of the per-domain
> >>> -value settable via Xen tools.
> >>> +Specify the default maximum number of frames to use as part of a domains
> >>> +maptrack array unless a different value is specified at domain creation.
> >>> +
> >>> +Dom0 is using this value for sizing its maptrack array.
> >>
> >> ... here. And even ordinary DomU-s appear to default to that in the
> >> absence of a specific value in the guest config. IOW at the very least
> >> the info you add should not be misleading. Better would be if the pre-
> >> existing info was adjusted at the same time.
> > 
> > Aren't domUs already clearly covered by the sentence:
> > 
> > "Specify the default maximum number of frames to use as part of a 
> > domains..."
> 
> Hmm, yes, my attention was caught too much by the Dom0 statement. While ...
> 
> > IMO dom0 needs to be explicitly mentioned because in that case the
> > value provided is not the one used by default, but rather the one that
> > gets used.
> 
> ... explicitly mentioning Dom0 is fine, I still think this needs wording
> differently here, because Dom0 doesn't actively do anything with this
> value (and, as said, it can't even obtain it other than by probing how
> many mappings it can create).
> 
> >> I also wonder about the specific wording down here: While the max grant
> >> table size can indeed be queried, this isn't the case for the maptrack
> >> array. A domain also doesn't need to know its size, so maybe "This value
> >> is used to size all domains' maptrack arrays, unless overridden by their
> >> guest config"?
> > 
> > I think the wording I've added already conveys this meaning:
> > 
> > "Specify the default maximum number of frames to use as part of a domains
> > maptrack array unless a different value is specified at domain creation."
> 
> Well, I mean specifically the Dom0 related statement.
> 
> Also to me "default maximum" reads odd (and slightly ambiguous). Would
> "default upper bound on the number of ..." perhaps be a little better?

So what about using:

"Specify the default upper bound on the number of frames which any
domain may use as part of its grant table unless a different value is
specified at domain creation.

Note this value is the enforced upper bound for dom0."

And similar for the maptrack option.

Thanks, Roger.