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

From: Ingo Molnar
Date: Thu Jul 21 2016 - 03:53:52 EST

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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@xxxxxxxxxx>
---
arch/x86/include/asm/bios_ebda.h | 2 +-
arch/x86/include/asm/x86_init.h | 4 +-
arch/x86/kernel/ebda.c | 124 +++++++++++++++++++++++++-------------
arch/x86/kernel/head32.c | 2 +-
arch/x86/kernel/head64.c | 2 +-
arch/x86/kernel/platform-quirks.c | 4 +-
6 files changed, 88 insertions(+), 50 deletions(-)

diff --git a/arch/x86/include/asm/bios_ebda.h b/arch/x86/include/asm/bios_ebda.h
index 2b00c776f223..4b7b8e71607e 100644
--- a/arch/x86/include/asm/bios_ebda.h
+++ b/arch/x86/include/asm/bios_ebda.h
@@ -17,7 +17,7 @@ static inline unsigned int get_bios_ebda(void)
return address; /* 0 means none */
}

-void reserve_ebda_region(void);
+void reserve_bios_regions(void);

#ifdef CONFIG_X86_CHECK_BIOS_CORRUPTION
/*
diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 4dcdf74dfed8..c519c052700a 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -168,14 +168,14 @@ struct x86_legacy_devices {
* struct x86_legacy_features - legacy x86 features
*
* @rtc: this device has a CMOS real-time clock present
- * @ebda_search: it's safe to search for the EBDA signature in the hardware's
+ * @reserve_bios_regions: it's safe to search for the EBDA signature in the hardware's
* low RAM
* @devices: legacy x86 devices, refer to struct x86_legacy_devices
* documentation for further details.
*/
struct x86_legacy_features {
int rtc;
- int ebda_search;
+ int reserve_bios_regions;
struct x86_legacy_devices devices;
};

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);
}
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index d784bb547a9d..2dda0bc4576e 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -26,7 +26,7 @@ static void __init i386_default_early_setup(void)
x86_init.resources.reserve_resources = i386_reserve_resources;
x86_init.mpparse.setup_ioapic_ids = setup_ioapic_ids_from_mpc;

- reserve_ebda_region();
+ reserve_bios_regions();
}

asmlinkage __visible void __init i386_start_kernel(void)
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index b72fb0b71dd1..99d48e7d2974 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -183,7 +183,7 @@ void __init x86_64_start_reservations(char *real_mode_data)
copy_bootdata(__va(real_mode_data));

x86_early_init_platform_quirks();
- reserve_ebda_region();
+ reserve_bios_regions();

switch (boot_params.hdr.hardware_subarch) {
case X86_SUBARCH_INTEL_MID:
diff --git a/arch/x86/kernel/platform-quirks.c b/arch/x86/kernel/platform-quirks.c
index b2f8a33b36ff..24a50301f150 100644
--- a/arch/x86/kernel/platform-quirks.c
+++ b/arch/x86/kernel/platform-quirks.c
@@ -7,12 +7,12 @@
void __init x86_early_init_platform_quirks(void)
{
x86_platform.legacy.rtc = 1;
- x86_platform.legacy.ebda_search = 0;
+ x86_platform.legacy.reserve_bios_regions = 0;
x86_platform.legacy.devices.pnpbios = 1;

switch (boot_params.hdr.hardware_subarch) {
case X86_SUBARCH_PC:
- x86_platform.legacy.ebda_search = 1;
+ x86_platform.legacy.reserve_bios_regions = 1;
break;
case X86_SUBARCH_XEN:
case X86_SUBARCH_LGUEST:

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