cbfstool:linux_trampoline: config CS and DS segment descriptors

The Linux trampoline code does not set up the segment descriptors for
__BOOT_CS and __BOOT_DS as described in the Linux kernel
documentation:

  ... a GDT must be loaded with the descriptors for selectors
  __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G
  flat segment; __BOOT_CS must have execute/read permission, and
  __BOOT_DS must have read/write permission;

This is not a problem when launching a Linux payload from coreboot, as
coreboot configures the segment descriptors at selectors 0x10 and
0x18.  Coreboot configures these selectors in the ramstage to match
what the Linux kernel expects (see
coreboot/src/arch/x86/lib/c_start.S).

When the cbfs payload is launched in other environments, SeaBIOS for
example, the segment descriptors are configured differently and the
cbfs Linux payload does not work.

If the cbfs Linux payload is to be used in multiple environments
should the trampoline needs to take care of the descriptors that Linux
requires.

This patch updates the Linux trampoline code to configure the 4G flat
descriptors that Linux expects.  The configuration is borrowed from
the descriptor configs in coreboot/src/arch/x86/lib/c_start.S for
selectors 0x10 and 0x18.

The linux_trampoline code is slightly refractored by defining the
trampoline entry address, 0x40000, as TRAMPOLINE_ENTRY_LOC.  This
definition is moved into a separate header file, linux_trampoline.h.
This header file is now included by both the trampoline assembly
language code and the trampoline loader C code.

The trampoline assembly language code can now use TRAMPOLINE_ENTRY_LOC
as scratch space for the sgdt CPU instruction.

Testing Done:

Verified the Linux payload is booted correctly in the following
environments:

1.  Coreboot -> Linux Payload

2.  Coreboot -> SeaBIOS -> Linux Payload: (previously did not work)

Change-Id: I888f74ff43073a6b7318f6713a8d4ecb804c0162
Signed-off-by: Curt Brune <curt@cumulusnetworks.com>
Reviewed-on: http://review.coreboot.org/6796
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/util/cbfstool/cbfs-payload-linux.c b/util/cbfstool/cbfs-payload-linux.c
index 33a5e1afb1..9e9a8741d0 100644
--- a/util/cbfstool/cbfs-payload-linux.c
+++ b/util/cbfstool/cbfs-payload-linux.c
@@ -302,9 +302,8 @@ int parse_bzImage_to_payload(const struct buffer *input,
 	                   PAYLOAD_SEGMENT_CODE, kernel_base);
 
 	/* trampoline */
-	uint64_t entrypoint = 0x40000; /*TODO: any better place? */
 	bzp_output_segment(&bzp, &bzp.trampoline,
-	                   PAYLOAD_SEGMENT_CODE, entrypoint);
+	                   PAYLOAD_SEGMENT_CODE, TRAMPOLINE_ENTRY_LOC);
 
 	/* cmdline */
 	bzp_output_segment(&bzp, &bzp.cmdline,
@@ -315,7 +314,7 @@ int parse_bzImage_to_payload(const struct buffer *input,
 	                   PAYLOAD_SEGMENT_DATA, initrd_base);
 
 	/* Terminating entry segment. */
-	bzp_output_segment(&bzp, NULL, PAYLOAD_SEGMENT_ENTRY, entrypoint);
+	bzp_output_segment(&bzp, NULL, PAYLOAD_SEGMENT_ENTRY, TRAMPOLINE_ENTRY_LOC);
 
 	/* Set size of buffer taking into account potential compression. */
 	buffer_set_size(&bzp.output, bzp.offset);