Re: [Xen-devel] [PATCH 2/4] xen: introduce grant_map_exists

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 2 Oct 2014, Jan Beulich wrote:
> >>> On 02.10.14 at 12:02, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > --- a/xen/common/grant_table.c
> > +++ b/xen/common/grant_table.c
> > @@ -484,6 +484,38 @@ static int _set_status(unsigned gt_version,
> >          return _set_status_v2(domid, readonly, mapflag, shah, act, status);
> >  }
> >  
> > +bool_t grant_map_exists(struct domain *ld,
> > +                        struct grant_table *rgt,
> > +                        unsigned long mfn)
> > +{
> > +    struct active_grant_entry *act;
> > +    grant_ref_t ref;
> > +    bool_t ret = 0;
> > +
> > +    spin_lock(&rgt->lock);
> > +
> > +    for ( ref = 0; ref != nr_grant_entries(rgt); ref++ )
> > +    {
> > +        act = &active_entry(rgt, ref);
> > +
> > +        if ( !act->pin )
> > +            continue;
> > +
> > +        if ( act->domid != ld->domain_id )
> > +            continue;
> > +
> > +        if ( act->frame != mfn )
> > +            continue;
> > +        
> > +        ret = 1;
> > +        break;
> > +    }
> > +
> > +    spin_unlock(&rgt->lock);
> > +
> > +    return ret;
> 
> By the time you get here the information you return is stale. Is that
> not a problem for the caller? And if it's not, why would it check in the
> first place?

This is just a security check to make sure that the caller is allowed to
cache flush the memory range. Even if it is a bit stale I think is OK:
it is not possible for the memory to have already been reused somewhere
else.

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