x86: Add .data section support for pre-memory stages

x86 pre-memory stages do not support the `.data` section and as a
result developers are required to include runtime initialization code
instead of relying on C global variable definition.

To illustrate the impact of this lack of `.data` section support, here
are two limitations I personally ran into:

1. The inclusion of libgfxinit in romstage for Raptor Lake has
   required some changes in libgfxinit to ensure data is initialized at
   runtime. In addition, we had to manually map some `.data` symbols in
   the `_bss` region.

2. CBFS cache is currently not supported in pre-memory stages and
   enabling it would require to add an initialization function and
   find a generic spot to call it.

Other platforms do not have that limitation. Hence, resolving it would
help to align code and reduce compilation based restriction (cf. the
use of `ENV_HAS_DATA_SECTION` compilation flag in various places of
coreboot code).

We identified three cases to consider:

1. eXecute-In-Place pre-memory stages
   - code is in SPINOR
   - data is also stored in SPINOR but must be linked in Cache-As-RAM
     and copied there at runtime

2. `bootblock` stage is a bit different as it uses Cache-As-Ram but
   the memory mapping and its entry code different

3. pre-memory stages loaded in and executed from
   Cache-As-RAM (cf. `CONFIG_NO_XIP_EARLY_STAGES`).

eXecute-In-Place pre-memory stages (#1) require the creation of a new
ELF segment as the code segment Virtual Memory Address and Load Memory
Address are identical but the data needs to be linked in
cache-As-RAM (VMA) but to be stored right after the code (LMA).

Here is the output `readelf --segments` on a `romstage.debug` ELF
binary.

    Program Headers:
      Type    Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
      LOAD    0x000080 0x02000000 0x02000000 0x21960 0x21960 R E 0x20
      LOAD    0x0219e0 0xfefb1640 0x02021960 0x00018 0x00018 RW  0x4

     Section to Segment mapping:
      Segment Sections...
       00     .text
       01     .data

Segment 0 `VirtAddr` and `PhysAddr` are at the same address while they
are totally different for the Segment 1 holding the `.data`
section. Since we need the data section `VirtAddr` to be in the
Cache-As-Ram and its `PhysAddr` right after the `.text` section, the
use of a new segment is mandatory.

`bootblock` (#2) also uses this new segment to store the data right
after the code and load it to Cache-As-RAM at runtime. However, the
code involved is different.

Not eXecute-In-Place pre-memory stages (#3) do not really need any
special work other than enabling a data section as the code and data
VMA / LMA translation vector is the same.

TEST=#1 and #2 verified on rex and qemu 32 and 64 bits:
     - The `bootblock.debug`, `romstage.debug` and
       `verstage.debug` all have data stored at the end of the `.text`
       section and code to copy the data content to the Cache-As-RAM.
     - The CBFS stages included in the final image has not improperly
       relocated any of the `.data` section symbol.
     - Test purposes global data symbols we added in bootblock,
       romstage and verstage are properly accessible at runtime
     #3: for "Intel Apollolake DDR3 RVP1" board, we verified that the
     generated romstage ELF includes a .data section similarly to a
     regular memory enabled stage.

Change-Id: I030407fcc72776e59def476daa5b86ad0495debe
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77289
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

2 files changed, 11 insertions, 6 deletions

diff --git a/src/include/rules.h b/src/include/rules.h
index ae8118ebb4..02c36f2b25 100644
--- a/src/include/rules.h
+++ b/src/include/rules.h
@@ -274,16 +274,16 @@
 #if ENV_X86
 /* Indicates memory layout is determined with arch/x86/car.ld. */
 #define ENV_CACHE_AS_RAM		(ENV_ROMSTAGE_OR_BEFORE && !CONFIG(RESET_VECTOR_IN_RAM))
-/* No .data sections with execute-in-place from ROM.  */
-#define ENV_HAS_DATA_SECTION	!ENV_CACHE_AS_RAM
 #else
-/* Both .data and .bss, sometimes SRAM not DRAM. */
-#define ENV_HAS_DATA_SECTION	1
 #define ENV_CACHE_AS_RAM		0
 #endif
 
-/* Indicates if the stage uses the _bss region defined in arch/x86/car.ld */
-#define ENV_SEPARATE_BSS	(ENV_CACHE_AS_RAM && (ENV_BOOTBLOCK || !CONFIG(NO_XIP_EARLY_STAGES)))
+/* Indicates .data section support. */
+#define ENV_HAS_DATA_SECTION		1
+
+/* Indicates if the stage uses the _data and _bss regions defined in
+ * arch/x86/car.ld */
+#define ENV_SEPARATE_DATA_AND_BSS	(ENV_CACHE_AS_RAM && (ENV_BOOTBLOCK || !CONFIG(NO_XIP_EARLY_STAGES)))
 
 /* Currently rmodules, ramstage and smm have heap. */
 #define ENV_HAS_HEAP_SECTION	(ENV_RMODULE || ENV_RAMSTAGE || ENV_SMM)
diff --git a/src/include/symbols.h b/src/include/symbols.h
index a03af08463..5410798f06 100644
--- a/src/include/symbols.h
+++ b/src/include/symbols.h
@@ -50,6 +50,11 @@ DECLARE_REGION(heap)
 DECLARE_REGION(asan_shadow)
 #endif
 
+#if ENV_SEPARATE_DATA_AND_BSS
+DECLARE_REGION(data)
+DECLARE_REGION(data_load)
+#endif
+
 /* Regions for execution units. */
 
 DECLARE_REGION(payload)