On Fri, 9 Aug 2013, Konrad Rzeszutek Wilk wrote:

quoted

+ * requested. The new pages are going to be "pinned": it's guaranteed
+ * that their p2m mapping won't be changed until explicitly "unpinned".

What if you try to balloon them out? What happens then? Does that
unpin them automatically?

What if I use said "pin" page for grants? Can they be shared with another
guest?

I'll mention that only normal guest r/w memory can be pinned.

quoted

+ * If return code is zero then @out.extent_list provides the MFNs of the
+ * newly-allocated memory.  Returns zero on complete success, otherwise
+ * a negative error code.

Ahem. Which ones? I know you didn't like the existing grant code b/c it
returned some general error. Would it make sense to say which are ones
are expected?

I'll do that.

quoted

+ * On complete success then always @nr_exchanged == @in.nr_extents.  On
+ * partial success @nr_exchanged indicates how much work was done.

And no error?

quoted

+ */
+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?

As per the other email, that goes in out.address_bits. I'll add a note.

quoted

+     */
+    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
+     */
+    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!
+     */
+    xen_ulong_t nr_exchanged;
+};
+DEFINE_GUEST_HANDLE_STRUCT(xen_get_dma_buf);
+
+#define XENMEM_put_dma_buf             27
+/*
+ * XENMEM_put_dma_buf unpins a set of pages, previously pinned by
+ * XENMEM_get_dma_buf. After this call the p2m mapping of the pages can
+ * be transparently changed by the hypervisor, as usual. The pages are
+ * still accessible from the guest.

I don't understand what 'pinned' means. What is a 'pinned' page versus
a normal page? Is this like the pagetables which must be _RO?

A pinned page is just a normal page that the hypervisor promises is
going to keep having the same machine address until unpinned.