[xen] Support scatter-gather to allow for jumbo frames

The use of jumbo frames for the Xen netfront virtual NIC requires the
use of scatter-gather ("feature-sg"), with the receive descriptor ring
becoming a list of page-sized buffers and the backend using as many
page buffers as required for each packet.

Since iPXE's abstraction of an I/O buffer does not include any sort of
scatter-gather list, this requires an extra allocation and copy on the
receive datapath for any packet that spans more than a single page.

This support is required in order to successfully boot an AWS EC2
virtual machine (with non-enhanced networking) via iSCSI if jumbo
frames are enabled, since the netback driver used in EC2 seems not to
allow "feature-sg" to be renegotiated once the Linux kernel driver
takes over.

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

src/include/ipxe/xengrant.h

@@ -166,16 +166,17 @@ xengrant_invalidate ( struct xen_hypervisor *xen, grant_ref_t ref ) {
  * @v ref		Grant reference
  * @v domid		Domain ID
  * @v subflags		Additional flags
- * @v page		Page start
+ * @v addr		Physical address within page
  * @ret rc		Return status code
  */
 static inline __attribute__ (( always_inline )) int
 xengrant_permit_access ( struct xen_hypervisor *xen, grant_ref_t ref,
-			 domid_t domid, unsigned int subflags, void *page ) {
+			 domid_t domid, unsigned int subflags,
+			 physaddr_t addr ) {
 	struct grant_entry_header *hdr = xengrant_header ( xen, ref );
 	struct grant_entry_v1 *v1 = xengrant_v1 ( hdr );
 	union grant_entry_v2 *v2 = xengrant_v2 ( hdr );
-	unsigned long frame = ( virt_to_phys ( page ) / PAGE_SIZE );
+	unsigned long frame = ( addr / PAGE_SIZE );
 
 	/* Fail (for test purposes) if applicable */
 	if ( ( XENGRANT_FAIL_RATE > 0 ) &&