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SubjectRe: [PATCH v3 07/10] xen: introduce XENMEM_get_dma_buf and xen_put_dma_buf
On Fri, 9 Aug 2013, Konrad Rzeszutek Wilk wrote:
> > > +struct xen_get_dma_buf {
> > > + /*
> > > + * [IN] Details of memory extents to be exchanged (GMFN bases).
> > > + * Note that @in.address_bits is ignored and unused.
> >
> > Ohhhh, why? What if the user wants to be it under 2G?
>
> Looking a bit more at the code revealed that we stick that in the
> @out.address_bits
> >
> > > + */
> > > + struct xen_memory_reservation in;
> > > +
> > > + /*
> > > + * [IN/OUT] Details of new memory extents.
> > > + * We require that:
> > > + * 1. @in.domid == @out.domid
> > > + * 2. @in.nr_extents << @in.extent_order ==
> > > + * @out.nr_extents << @out.extent_order
> > > + * 3. @in.extent_start and @out.extent_start lists must not overlap
> > > + * 4. @out.extent_start lists GPFN bases to be populated
> > > + * 5. @out.extent_start is overwritten with allocated GMFN bases
>
> .. which you should document, otherwise the hypervisor might try to give
> you pages under 2^0..

Yes, you are right. I'll add a note about out.address_bits.


> > > + */
> > > + struct xen_memory_reservation out;
> > > +
> > > + /*
> > > + * [OUT] Number of input extents that were successfully exchanged:
> > > + * 1. The first @nr_exchanged input extents were successfully
> > > + * deallocated.
> > > + * 2. The corresponding first entries in the output extent list correctly
> > > + * indicate the GMFNs that were successfully exchanged.
> > > + * 3. All other input and output extents are untouched.
> > > + * 4. If not all input exents are exchanged then the return code of this
> > > + * command will be non-zero.
> > > + * 5. THIS FIELD MUST BE INITIALISED TO ZERO BY THE CALLER!
>
> as this says the initial value is zero.

This is referring to nr_exchanged.


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