Re: [PATCH] xen: add option to disable GNTTABOP_transfer

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.02.2022 10:02, Juergen Gross wrote:
> The grant table operation GNTTABOP_transfer is meant to be used in
> PV device backends, and it hasn't been used in Linux since the old
> Xen-o-Linux days.

Kind of unusual spelling of XenoLinux ;-)

> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -181,6 +181,7 @@ static int parse_gnttab_max_maptrack_frames(const char 
> *arg)
>  
>  unsigned int __read_mostly opt_gnttab_max_version = GNTTAB_MAX_VERSION;
>  static bool __read_mostly opt_transitive_grants = true;
> +static bool __read_mostly opt_grant_transfer = true;

If this was conditional upon PV (with a #define to false in the
opposite case), it could be __ro_after_init right away, while at
the same time allowing the compiler to eliminate gnttab_transfer().

> @@ -204,6 +205,8 @@ static int __init parse_gnttab(const char *s)
>          }
>          else if ( (val = parse_boolean("transitive", s, ss)) >= 0 )
>              opt_transitive_grants = val;
> +        else if ( (val = parse_boolean("transfer", s, ss)) >= 0 )
> +            opt_grant_transfer = val;
>          else
>              rc = -EINVAL;

To possibly save a further roundtrip: If the PV dependency was added
above, I'd like to ask to follow the model of parse_iommu_param()
here and use "#ifndef opt_grant_transfer" around the added code in
favor of "#ifdef CONFIG_PV".

> @@ -2233,6 +2236,9 @@ gnttab_transfer(
>      unsigned int max_bitsize;
>      struct active_grant_entry *act;
>  
> +    if ( !opt_grant_transfer )
> +        return -ENOSYS;

-EOPNOTSUPP please.

Jan