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

[Juergen Gross <jgross@xxxxxxxx>]

On 03.02.22 10:10, Jan Beulich wrote:
> 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().
Nice idea. The other option would be to put all (or most) of
gnttab_transfer() in a "#ifdef CONFIG_PV" section, allowing to
remove the "#ifdef CONFIG_X86" parts in it, too.

> > @@ -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".
Okay.

> > @@ -2233,6 +2236,9 @@ gnttab_transfer(
> >       unsigned int max_bitsize;
> >       struct active_grant_entry *act;
> >   
> > +    if ( !opt_grant_transfer )
> > +        return -ENOSYS;
> -EOPNOTSUPP please.
Yes, that's better.


Juergen

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