Re: [PATCH v2] xen/grants: repurpose command line max options

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2023 15:45, Roger Pau Monne wrote:
> Slightly change the meaning of the command line
> gnttab_max_{maptrack_,}frames: do not use them as upper bounds for the
> passed values at domain creation, instead just use them as defaults
> in the absence of any provided value.
> 
> It's not very useful for the options to be used both as defaults and
> as capping values for domain creation inputs.  The defaults passed on
> the command line are used by dom0 which has a very different grant
> requirements than a regular domU.  dom0 usually needs a bigger
> maptrack array, while domU usually require a bigger number of grant
> frames.
> 
> The relaxation in the logic for the maximum size of the grant and
> maptrack table sizes doesn't change the fact that domain creation
> hypercall can cause resource exhausting, so disaggregated setups
> should take it into account.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit perhaps with yet one more wording change (which I'd be happy to
make while committing, provided you agree):

> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1232,11 +1232,11 @@ 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 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.
>  
> -Dom0 is using this value for sizing its grant table.
> +Note this value is the effective upper bound for dom0.

DomU-s created during Xen boot are equally taking this as their effective
value, aiui. So I'd be inclined to amend this: "... for dom0, and for
any domU created in the course of Xen booting".

> @@ -1245,9 +1245,11 @@ 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 upper bound on the number of frames which any domain may
> +use as part of its maptrack array unless a different value is specified at
> +domain creation.
> +
> +Note this value is the effective upper bound for dom0.

Same here then of course (which actually is bad, because those DomU-s
shouldn't need this big a maptrack table, but that's a separate topic).

Jan