[xen] Use version 1 grant tables by default

Using version 1 grant tables limits guests to using 16TB of grantable
RAM, and prevents the use of subpage grants.  Some versions of the Xen
hypervisor refuse to allow the grant table version to be set after the
first grant references have been created, so the loaded operating
system may be stuck with whatever choice we make here.  We therefore
currently use version 2 grant tables, since they give the most
flexibility to the loaded OS.

Current versions (7.2.0) of the Windows PV drivers have no support for
version 2 grant tables, and will merrily create version 1 entries in
what the hypervisor believes to be a version 2 table.  This causes
some confusion.

Avoid this problem by attempting to use version 1 tables, since
otherwise we may render Windows unable to boot.

Play nicely with other potential bootloaders by accepting either
version 1 or version 2 grant tables (if we are unable to set our
requested version).

Note that the use of version 1 tables on a 64-bit system introduces a
possible failure path in which a frame number cannot fit into the
32-bit field within the v1 structure.  This in turn introduces
additional failure paths into netfront_transmit() and
netfront_refill_rx().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/include/ipxe/xen.h b/src/include/ipxe/xen.h
index 546b0c348..60aabe03e 100644
--- a/src/include/ipxe/xen.h
+++ b/src/include/ipxe/xen.h
@@ -27,9 +27,11 @@ struct xen_hypercall;
 /** A Xen grant table */
 struct xen_grant {
 	/** Grant table entries */
-	union grant_entry_v2 *table;
-	/** Number of grant table entries (must be a power of two) */
-	unsigned int count;
+	struct grant_entry_v1 *table;
+	/** Total grant table length */
+	size_t len;
+	/** Entry size shift (for later version tables) */
+	unsigned int shift;
 	/** Number of grant table entries in use */
 	unsigned int used;
 	/** Most recently used grant reference */