Re: [Xen-devel] netif & grant tables

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> You are right, it's not the grant tables per se that need the privileged
> bit to be set, but other functions need it and keep backends from working.
>
> Here are two code paths from the network backend:

I suspected it might be something like this.  There's no reason for those to 
require privilege either: they can fairly trivially be converted to use grant 
tables too.  Once it's fully grant-table-ified it shouldn't be necessary to 
make such domains be privileged.

Full grant tables support also a pre-req for the point to point "snappable 
frontend" connections directly between domains with high bandwidth 
requirements.

Cheers,
Mark

> from netback/interface.c calls
>
> linux-2.6.11-xen-sparse/arch/xen/i386/mm/ioremap.c:direct_remap_area_pages(
>) calls
>       HYPERVISOR_mmu_update calls
>          xen/arch/x86/mm.c:do_mmu_update calls
>            set_foreigndom() which has an IS_PRIV() in the path
>
> -> The direct_remap_area_pages call fails if a domain does not have the
> privilege bit set.
>
>
> netback/netback.c: alloc_mfn() calls
>    HYPERVISOR_dom_mem_op(MEMOP_increase_reservation, mfn_list,
> MAX_MFN_ALLOC, 0);
>      xen/common/dom_mem_ops.c:do_dom_mem_op() is called which has a
> IS_PRIV() in the path
>
>
>
>  Stefan
>
> > What I meant was that since grant tables are an explicit capability (you
>
> can
>
> > only map a page of another dom if it gave you an explicit grant) there's
>
> no
>
> > need for mappings in the IO path to require special privileges at all.
>
> If
>
> > someone gave you a grant, they must trust you enough to access that
>
> page.
>
> > Cheers,
> > Mark
> >
> > >     Stefan
> > >
> > > > Cheers,
> > > > Mark
> > > >
> > > > > The privilege does so far not
> > > > > only mean to do dom 0 ops, but seems to also limit guest domains
>
> of
>
> > > doing
> > >
> > > > > other things - like the backend problem I see. I agree, though,
>
> that
>
> > > for
> > >
> > > > > grant table support a backend should not need privileges.
> > > > >
> > > > > > Cheers,
> > > > > > Mark
> > > > >
> > > > > Cheers,
> > > > >    Stefan
> > > >
> > > > _______________________________________________
> > > > Xen-devel mailing list
> > > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > > http://lists.xensource.com/xen-devel

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