x86/boot: Reorganize and clean up the BIOS area reservation code

So the reserve_ebda_region() code has accumulated a number of
problems over the years that make it really difficult to read
and understand:

- The calculation of 'lowmem' and 'ebda_addr' is an unnecessarily
  interleaved mess of first lowmem, then ebda_addr, then lowmem tweaks...

- 'lowmem' here means 'super low mem' - i.e. 16-bit addressable memory. In other
  parts of the x86 code 'lowmem' means 32-bit addressable memory... This makes it
  super confusing to read.

- It does not help at all that we have various memory range markers, half of which
  are 'start of range', half of which are 'end of range' - but this crucial
  property is not obvious in the naming at all ... gave me a headache trying to
  understand all this.

- Also, the 'ebda_addr' name sucks: it highlights that it's an address (which is
  obvious, all values here are addresses!), while it does not highlight that it's
  the _start_ of the EBDA region ...

- 'BIOS_LOWMEM_KILOBYTES' says a lot of things, except that this is the only value
  that is a pointer to a value, not a memory range address!

- The function name itself is a misnomer: it says 'reserve_ebda_region()' while
  its main purpose is to reserve all the firmware ROM typically between 640K and
  1MB, while the 'EBDA' part is only a small part of that ...

- Likewise, the paravirt quirk flag name 'ebda_search' is misleading as well: this
  too should be about whether to reserve firmware areas in the paravirt case.

- In fact thinking about this as 'end of RAM' is confusing: what this function
  *really* wants to reserve is firmware data and code areas! Once the thinking is
  inverted from a mixed 'ram' and 'reserved firmware area' notion to a pure
  'reserved area' notion everything becomes a lot clearer.

To improve all this rewrite the whole code (without changing the logic):

- Firstly invert the naming from 'lowmem end' to 'BIOS reserved area start'
  and propagate this concept through all the variable names and constants.

	BIOS_RAM_SIZE_KB_PTR		// was: BIOS_LOWMEM_KILOBYTES

	BIOS_START_MIN			// was: INSANE_CUTOFF

	ebda_start			// was: ebda_addr
	bios_start			// was: lowmem

	BIOS_START_MAX			// was: LOWMEM_CAP

- Then clean up the name of the function itself by renaming it
  to reserve_bios_regions() and renaming the ::ebda_search paravirt
  flag to ::reserve_bios_regions.

- Fix up all the comments (fix typos), harmonize and simplify their
  formulation and remove comments that become unnecessary due to
  the much better naming all around.

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 81 insertions, 43 deletions

diff --git a/arch/x86/kernel/ebda.c b/arch/x86/kernel/ebda.c
index afe65dffee80..6219eef20e2e 100644
--- a/arch/x86/kernel/ebda.c
+++ b/arch/x86/kernel/ebda.c
@@ -6,66 +6,104 @@
 #include <asm/bios_ebda.h>
 
 /*
+ * This function reserves all conventional PC system BIOS related
+ * firmware memory areas (some of which are data, some of which
+ * are code), that must not be used by the kernel as available
+ * RAM.
+ *
  * The BIOS places the EBDA/XBDA at the top of conventional
  * memory, and usually decreases the reported amount of
- * conventional memory (int 0x12) too. This also contains a
- * workaround for Dell systems that neglect to reserve EBDA.
- * The same workaround also avoids a problem with the AMD768MPX
- * chipset: reserve a page before VGA to prevent PCI prefetch
- * into it (errata #56). Usually the page is reserved anyways,
- * unless you have no PS/2 mouse plugged in.
+ * conventional memory (int 0x12) too.
+ *
+ * This means that as a first approximation on most systems we can
+ * guess the reserved BIOS area by looking at the low BIOS RAM size
+ * value and assume that everything above that value (up to 1MB) is
+ * reserved.
+ *
+ * But life in firmware country is not that simple:
+ *
+ * - This code also contains a quirk for Dell systems that neglect
+ *   to reserve the EBDA area in the 'RAM size' value ...
+ *
+ * - The same quirk also avoids a problem with the AMD768MPX
+ *   chipset: reserve a page before VGA to prevent PCI prefetch
+ *   into it (errata #56). (Usually the page is reserved anyways,
+ *   unless you have no PS/2 mouse plugged in.)
+ *
+ * - Plus paravirt systems don't have a reliable value in the
+ *   'BIOS RAM size' pointer we can rely on, so we must quirk
+ *   them too.
+ *
+ * Due to those various problems this function is deliberately
+ * very conservative and tries to err on the side of reserving
+ * too much, to not risk reserving too little.
+ *
+ * Losing a small amount of memory in the bottom megabyte is
+ * rarely a problem, as long as we have enough memory to install
+ * the SMP bootup trampoline which *must* be in this area.
  *
- * This functions is deliberately very conservative.  Losing
- * memory in the bottom megabyte is rarely a problem, as long
- * as we have enough memory to install the trampoline.  Using
- * memory that is in use by the BIOS or by some DMA device
- * the BIOS didn't shut down *is* a big problem.
+ * Using memory that is in use by the BIOS or by some DMA device
+ * the BIOS didn't shut down *is* a big problem to the kernel,
+ * obviously.
  */
 
-#define BIOS_LOWMEM_KILOBYTES	0x413
-#define LOWMEM_CAP		0x9f000U	/* Absolute maximum */
-#define INSANE_CUTOFF		0x20000U	/* Less than this = insane */
+#define BIOS_RAM_SIZE_KB_PTR	0x413
 
-void __init reserve_ebda_region(void)
+#define BIOS_START_MIN		0x20000U	/* 128K, less than this is insane */
+#define BIOS_START_MAX		0x9f000U	/* 640K, absolute maximum */
+
+void __init reserve_bios_regions(void)
 {
-	unsigned int lowmem, ebda_addr;
+	unsigned int bios_start, ebda_start;
 
 	/*
-	 * To determine the position of the EBDA and the
-	 * end of conventional memory, we need to look at
-	 * the BIOS data area. In a paravirtual environment
-	 * that area is absent. We'll just have to assume
-	 * that the paravirt case can handle memory setup
-	 * correctly, without our help.
+	 * NOTE: In a paravirtual environment the BIOS reserved
+	 * area is absent. We'll just have to assume that the
+	 * paravirt case can handle memory setup correctly,
+	 * without our help.
 	 */
-	if (!x86_platform.legacy.ebda_search)
+	if (!x86_platform.legacy.reserve_bios_regions)
 		return;
 
-	/* end of low (conventional) memory */
-	lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
-	lowmem <<= 10;
-
-	/* start of EBDA area */
-	ebda_addr = get_bios_ebda();
+	/* Get the start address of the EBDA page: */
+	ebda_start = get_bios_ebda();
 
 	/*
-	 * Note: some old Dells seem to need 4k EBDA without
-	 * reporting so, so just consider the memory above 0x9f000
-	 * to be off limits (bugzilla 2990).
+	 * Quirk: some old Dells seem to have a 4k EBDA without
+	 * reporting so in their BIOS RAM size value, so just
+	 * consider the memory above 640K to be off limits
+	 * (bugzilla 2990).
+	 *
+	 * We detect this case by filtering for nonsensical EBDA
+	 * addresses below 128K, where we can assume that they
+	 * are bogus and bump it up to a fixed 640K value:
 	 */
+	if (ebda_start < BIOS_START_MIN)
+		ebda_start = BIOS_START_MAX;
 
-	/* If the EBDA address is below 128K, assume it is bogus */
-	if (ebda_addr < INSANE_CUTOFF)
-		ebda_addr = LOWMEM_CAP;
+	/*
+	 * BIOS RAM size is encoded in kilobytes, convert it
+	 * to bytes to get a first guess at where the BIOS
+	 * firmware area starts:
+	 */
+	bios_start = *(unsigned short *)__va(BIOS_RAM_SIZE_KB_PTR);
+	bios_start <<= 10;
 
-	/* If lowmem is less than 128K, assume it is bogus */
-	if (lowmem < INSANE_CUTOFF)
-		lowmem = LOWMEM_CAP;
+	/*
+	 * If bios_start is less than 128K, assume it is bogus
+	 * and bump it up to 640K:
+	 */
+	if (bios_start < BIOS_START_MIN)
+		bios_start = BIOS_START_MAX;
 
-	/* Use the lower of the lowmem and EBDA markers as the cutoff */
-	lowmem = min(lowmem, ebda_addr);
-	lowmem = min(lowmem, LOWMEM_CAP); /* Absolute cap */
+	/*
+	 * Use the lower of the bios_start and ebda_start
+	 * as the starting point, but don't allow it to
+	 * go beyond 640K:
+	 */
+	bios_start = min(bios_start, ebda_start);
+	bios_start = min(bios_start, BIOS_START_MAX);
 
-	/* reserve all memory between lowmem and the 1MB mark */
-	memblock_reserve(lowmem, 0x100000 - lowmem);
+	/* Reserve all memory between bios_start and the 1MB mark: */
+	memblock_reserve(bios_start, 0x100000 - bios_start);
 }